Add an assert preventing buffer overflows

Make sure that the workspace can accomodate the pipelined data before
memmoving it into place.

Add a comment on an open issue in the H2 code path that could trigger
this assert.

bin/varnishd/cache/cache_session.c

@@ -196,6 +196,7 @@ HTC_RxInit(struct http_conn *htc, struct ws *ws)
 	if (htc->pipeline_b != NULL) {
 		l = htc->pipeline_e - htc->pipeline_b;
 		assert(l > 0);
+		assert(l <= ws->r - htc->rxbuf_b);
 		memmove(htc->rxbuf_b, htc->pipeline_b, l);
 		htc->rxbuf_e += l;
 		htc->pipeline_b = NULL;

bin/varnishd/http2/cache_http2_proto.c

@@ -661,6 +661,9 @@ h2_new_ou_session(struct worker *wrk, struct h2_sess *h2,
 	h2->htc->pipeline_e = req->htc->pipeline_e;
 	req->htc->pipeline_b = NULL;
 	req->htc->pipeline_e = NULL;
+	/* XXX: This call may assert on buffer overflow if the pipelined
+	   data exceeds the available space in the aws workspace. What to
+	   do about the overflowing data is an open issue. */
 	HTC_RxInit(h2->htc, wrk->aws);
 
 	/* Start req thread */