Do not pass NULL for the string in catch_errors

Do not pass NULL for the string in catch_errors

I caught a segmentation fault while running gdb.reverse/sigall-reverse.exp,
in a mingw32 GDB, in this code path. It boils down to the code trying to
strlen () a NULL pointer. I tracked things down and it looks like
record_full_message_wrapper_safe is the only offender.

gdb/ChangeLog:

2015-10-26  Luis Machado  <lgustavo@codesourcery.com>

	* record-full.c (record_full_message_wrapper_safe): Pass empty string to
	catch_errors call instead of NULL.