restart in vcl_deliver{} would crash in vcl_fetch{} due to missing cleanup.

Found & Fixed by:	Martin

Fixes	#979

bin/varnishd/cache_center.c

@@ -233,6 +233,7 @@ cnt_prepresp(struct sess *sp)
 		AZ(sp->obj);
 		sp->restarts++;
 		sp->director = NULL;
+		sp->wrk->h_content_length = NULL;
 		http_Setup(sp->wrk->bereq, NULL);
 		http_Setup(sp->wrk->beresp, NULL);
 		http_Setup(sp->wrk->resp, NULL);

bin/varnishtest/tests/r00979.vtc

+varnishtest "r00979.vtc Test restart when do_stream in vcl_deliver"
+
+server s1 {
+	rxreq
+	txresp -status 200 -body "1"
+	expect_close
+
+	accept
+	rxreq
+	txresp -status 200 -body "11"
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_fetch {
+		set beresp.do_stream = true;
+	}
+	sub vcl_deliver {
+		if (req.restarts == 0) {
+			return (restart);
+		}
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.bodylen == 2
+} -run