selftests:timers: posix_timers: Fix warn_unused_result in __fatal_error()

__fatal_error routine doesn't check strerror_r() return value,
which results in the following compile time warning:

posix_timers.c: In function ‘__fatal_error’:
posix_timers.c:31:9: warning: ignoring return value of ‘strerror_r’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   31 |         strerror_r(errno, buf, sizeof(buf));

Fix this by adding a check for return value and error handling appropriate
for the GNU-specific strerror_r() in use in __fatal_error(). Check if
return string is null and handle accordingly.

From Linux strerror_r() manual page:

"The GNU-specific strerror_r() returns a pointer to a string containing
the error message. This may be either a pointer to a string that the
function stores in buf, or a pointer to some (immutable) static string
(in which case buf is unused). If the function stores a string in buf,
then at most buflen bytes are stored (the string may be truncated if
buflen is too small and errnum is unknown). The string always includes
a terminating null byte."

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: John Stultz <jstultz@google.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>