dont panic for NULL backend

src/vmod_shard.c

@@ -127,7 +127,13 @@ vmod_shard_add_backend(VRT_CTX, struct vmod_shard_shard *vshard,
     VCL_BACKEND be, VCL_STRING ident, VCL_DURATION rampup)
 {
 	CHECK_OBJ_NOTNULL(vshard, VMOD_SHARD_SHARD_MAGIC);
-	AN(be);
+
+	if (be == NULL) {
+		shard_err0(ctx, vshard->shardd,
+		    ".backend_add() NULL backend given");
+		return 0;
+	}
+
 	return shardcfg_add_backend(ctx, priv, vshard->shardd,
 	    be, ident, rampup);
 }