Introduce a HTC_S_JUNK return value

bin/varnishd/cache/cache.h

@@ -1000,6 +1000,7 @@ task_func_t SES_Proto_Sess;
 task_func_t SES_Proto_Req;
 
 enum htc_status_e {
+	HTC_S_JUNK =		-5,
 	HTC_S_CLOSE =		-4,
 	HTC_S_TIMEOUT =		-3,
 	HTC_S_OVERFLOW =	-2,

bin/varnishd/cache/cache_session.c

@@ -259,6 +259,10 @@ SES_RxReq(const struct worker *wrk, struct req *req, htc_complete_f *func)
 			return (HTC_S_OVERFLOW);
 		}
 		hs = func(req->htc);
+		if (hs == HTC_S_JUNK) {
+			WS_ReleaseP(req->htc->ws, req->htc->rxbuf_b);
+			return (HTC_S_JUNK);
+		}
 		if (hs == HTC_S_COMPLETE) {
 			/* Got it, run with it */
 			if (isnan(req->t_first))