Fix ubsan build

Fix ubsan build

I tried a build using the undefined behavior sanitizer, and gcc gave
this error:

In file included from /usr/include/string.h:495,
                 from ../gnulib/import/string.h:41,
                 from ../../binutils-gdb/gdb/../gdbsupport/common-defs.h:95,
                 from ../../binutils-gdb/gdb/nat/linux-osdata.c:20:
In function 'char* strncpy(char*, const char*, size_t)',
    inlined from 'void time_from_time_t(char*, int, TIME_T)' at ../../binutils-gdb/gdb/nat/linux-osdata.c:923:15,
    inlined from 'void time_from_time_t(char*, int, TIME_T)' at ../../binutils-gdb/gdb/nat/linux-osdata.c:911:1,
    inlined from 'void linux_xfer_osdata_sem(buffer*)' at ../../binutils-gdb/gdb/nat/linux-osdata.c:1082:22:
/usr/include/bits/string_fortified.h:106:34: error: 'char* __builtin_strncpy(char*, const char*, long unsigned int)' specified bound 32 equals destination size [-Werror=stringop-truncation]

This patch fixes the problem by subtracting one from the length
parameter to strncpy.

I changed a couple of other similar functions -- gcc does not warn
about these, but I didn't see any substantial difference between the
different cases, and I think these are just latent warnings, to be
triggered in the future by a change to inlining heuristics.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tromey@adacore.com>

	* nat/linux-osdata.c (user_from_uid, time_from_time_t)
	(group_from_gid): Subtract one from strncpy length.