virutil: Resolve Coverity RESOURCE_LEAK

This ends up being a very bizarre false positive. With an assist from
eblake, the claim is that mgetgroups() could return a -1 value, but yet
still have a groups buffer allocated, yet the example shown doesn't
seem to prove that.

Rather than fret about it, by adding a well placed sa_assert() on the
returned *list value we can "assure" ourselves that the mgetgroups()
failure path won't signal this condition.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/util/virutil.c

@@ -1063,6 +1063,7 @@ virGetGroupList(uid_t uid, gid_t gid, gid_t **list)
 
     ret = mgetgroups(user, primary, list);
     if (ret < 0) {
+        sa_assert(!*list);
         virReportSystemError(errno,
                              _("cannot get group list for '%s'"), user);
         goto cleanup;