socket-util: fix getpeergroups() assert(fd) (#8080)

Don't assert on zero-value fds.

Fixes #8075.

src/basic/socket-util.c

@@ -1008,7 +1008,7 @@ int getpeergroups(int fd, gid_t **ret) {
         socklen_t n = sizeof(gid_t) * 64;
         _cleanup_free_ gid_t *d = NULL;
 
-        assert(fd);
+        assert(fd >= 0);
         assert(ret);
 
         for (;;) {