Proper locking around n_backend. Found by Coverity.

vmod/vmod_directors.c

@@ -94,9 +94,11 @@ vdir_release(struct vdir *vd)
 
 	CHECK_OBJ_NOTNULL(vd, VDIR_MAGIC);
 
+	vdir_wrlock(vd);
 	for (u = 0; u < vd->n_backend; u++)
 		VRT_Assign_Backend(&vd->backend[u], NULL);
 	vd->n_backend = 0;
+	vdir_unlock(vd);
 }
 
 void

vmod/vmod_directors_round_robin.c

@@ -90,9 +90,9 @@ vmod_rr_resolve(VRT_CTX, VCL_BACKEND dir)
 			break;
 	}
 	rr->nxt = nxt;
-	vdir_unlock(rr->vd);
 	if (u == rr->vd->n_backend)
 		be = NULL;
+	vdir_unlock(rr->vd);
 	return (be);
 }