Fail gracefully if we can't get a backend on pipe

Fixes #1730

bin/varnishd/cache/cache_backend.c

@@ -277,6 +277,10 @@ vbe_dir_http1pipe(const struct director *d, struct req *req, struct busyobj *bo)
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 
 	i = vbe_dir_getfd(d, bo);
+	if (i < 0) {
+		VSLb(bo->vsl, SLT_FetchError, "no backend connection");
+		return;
+	}
 	V1P_Process(req, bo, i);
 	vbe_dir_finish(d, bo->wrk, bo);
 }

bin/varnishtest/tests/r01730.vtc

+varnishtest "Test connection error on pipe"
+
+varnish v1 -vcl {
+       backend default { .host = "${bad_ip}"; }
+       sub vcl_recv {
+               return (pipe);
+       }
+} -start
+
+client c1 {
+       txreq
+       expect_close
+} -run
+
+varnish v1 -expect sc_rx_timeout == 1