Don't panic on malformed Vary headers.

Fixes: #1275

Test case by: Dag Haavi Finstad

bin/varnishd/cache/cache_vary.c

@@ -130,7 +130,11 @@ VRY_Create(struct req *req, const struct http *hp, struct vsb **psb)
 			q++;
 		if (*q == '\0')
 			break;
-		xxxassert(*q == ',');
+		if (*q != ',') {
+			VSLb(req->vsl, SLT_Error, "Malformed Vary header");
+			error = 1;
+			break;
+		}
 		p = q;
 	}
 

bin/varnishtest/tests/r01275.vtc

+varnishtest "#1275 - panic with malformed Vary header"
+
+server s1 {
+       rxreq
+       txresp -hdr "Vary: foo bar"
+} -start
+
+varnish v1 -vcl+backend { } -start
+
+client c1 {
+       txreq
+       rxresp
+       expect resp.status == 503
+} -run