This patch fixes a "set but not used" warning in posix/bug-regex31.c.
A variable res stored an indication of whether the test behaved as
expected, but was then ignored and the test returned 0 unconditionally
(as an mtrace test, the auxiliary test for leaks could still have
failed if that bug was present). This patch makes the test return
res.
Tested for x86_64.
* posix/bug-regex31.c (main): Return RES not 0.
