Fix: lib-logging: possible buffer not null terminated

Fix: lib-logging: possible buffer not null terminated

Issue
=====
While fixing Coverity warnings, commit 879e14e8 introduced other
warnings. Indeed, the destination string after the `strncpy()` call
would be left not null terminated if the size of the source buffer is
equal or larger than the `TMP_PREFIX_LEN`. Also, the `strncat()` does
not consider the characters added by the previous `strncpy()` call when
supplying the size of the destination buffer. This could result in
buffer overflows.

Solution
========
Use `snprintf()` to concatenate the two strings and set the last
character to `\0`. It's worth it to make the code easier to understand
even though the `snprintf()` is probably more work because of the format
string.

Drawbacks
=========
None.

Notes
=====
List of coverity defect reports:
	CID 1401510 (#5 of 5): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 19. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401511 (#2 of 2): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 4. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401513 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 19. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401517 (#5 of 5): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 9. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	ID 1401519 (#4 of 4): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 15. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401523 (#3 of 3): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 16. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401525 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 10. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401526 (#2 of 2): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 11. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401528 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 15. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401532 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 5. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401533 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 6. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401534 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 12. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401535 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 13. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401537 (#2 of 2): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 23. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401539 (#10 of 10): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 10. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401540 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 28. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

	CID 1401541 (#1 of 1): Buffer not null terminated
	(BUFFER_SIZE_WARNING) 9. buffer_size_warning: Calling strncpy
	with a maximum size argument of 64 bytes on destination array
	tmp_prefix of size 64 bytes might leave the destination string
	unterminated.

Reported-by: Coverity - Buffer not null terminated
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie10d4d6c8b1a0caff2fe70bbb1046673bbb1a999
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1333
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>