Check fgets(3) return value

Check fgets(3) return value

When fgets() function fails, contents of the buffer is undefined.  That
is, fgets() return value needs to be checked, to avoid undefined behavior.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>