getopt: silence clang warning

getopt: silence clang warning

http://sourceware.org/bugzilla/show_bug.cgi?id=12922

Only triggered by a program that uses "W;" in the short opt string
without providing a longopts array; that can be argued to be a
programmer error, so gnulib won't bother replacing glibc getopt if
it has that bug.

* lib/getopt.c (_getopt_internal_r): Avoid unlikely NULL
dereference.
Reported by Gustavo Martin Domato.

Signed-off-by: Eric Blake <eblake@redhat.com>