Avoid stringop-truncation errors

Avoid stringop-truncation errors

I configured with -fsanitize=address and built gdb.  linux-tdep.c and
ada-tasks.c failed to build due to some stringop-truncation errors,
e.g.:

In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘int linux_fill_prpsinfo(elf_internal_linux_prpsinfo*)’ at ../../binutils-gdb/gdb/linux-tdep.c:1742:11,
    inlined from ‘char* linux_make_corefile_notes(gdbarch*, bfd*, int*)’ at ../../binutils-gdb/gdb/linux-tdep.c:1878:27:
/usr/include/bits/string_fortified.h:106:34: error: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 81 equals destination size [-Werror=stringop-truncation]

This patch fixes the problem by using "sizeof - 1" in the call to
strndup, as recommended in the GCC manual.  This doesn't make a
difference here because the next line, in all cases, sets the final
element to '\0' anyway.

gdb/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* ada-tasks.c (read_atcb): Use smaller length in strncpy call.
	* linux-tdep.c (linux_fill_prpsinfo): Use smaller length in
	strncpy call.