[PATCH 1/2] selftests:timers: posix_timers: Fix warn_unused_result in __fatal_error()

[Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>]

__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.