string: strerror must not return NULL (bug 30555)

For strerror, this fixes commit 28aff047 ("string:
Implement strerror in terms of strerror_l").  This commit avoids
returning NULL for strerror_l as well, although POSIX allows this
behavior for strerror_l.

Reviewed-by: Arjun Shankar <arjun@redhat.com>