close a potential race which could cause an out-of-bounds array access

We're only holding a read lock on the director, but we're updating
the nxt member concurrently. This should be acceptable as a performance
tradeoff - the only consequence is that round-robin is not strictly
going around - it may occasionally skip a backend or hand out the same
multiple times in a row.

the race is:

	thread	code

	A:	rr->nxt %= rr->vd->n_backend;
	// rr->nxt == rr->vd->n_backend - 1
	B:	rr->nxt++;
	// rr->nxt == rr->vd->n_backend
	A:	be = rr->vd->backend[nxt];
	// BOOM

should fix #2024

lib/libvmod_directors/round_robin.c

@@ -62,15 +62,16 @@ vmod_rr_resolve(const struct director *dir, struct worker *wrk,
 	struct vmod_directors_round_robin *rr;
 	unsigned u;
 	VCL_BACKEND be = NULL;
+	unsigned nxt;
 
 	CHECK_OBJ_NOTNULL(dir, DIRECTOR_MAGIC);
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 	CAST_OBJ_NOTNULL(rr, dir->priv, VMOD_DIRECTORS_ROUND_ROBIN_MAGIC);
 	vdir_rdlock(rr->vd);
 	for (u = 0; u < rr->vd->n_backend; u++) {
-		rr->nxt %= rr->vd->n_backend;
-		be = rr->vd->backend[rr->nxt];
+		nxt = rr->nxt %= rr->vd->n_backend;
+		be = rr->vd->backend[nxt];
 		rr->nxt++;
 		CHECK_OBJ_NOTNULL(be, DIRECTOR_MAGIC);
 		if (be->healthy(be, bo, NULL))