Use the idle read timeout only on empty requests

We only want to return the connection early to the waiter when the
request is empty. Correct the read timeout calculation to reflect
that.

Thanks to Stackpath for helping to debug this issue.

Fixes: #2492

bin/varnishd/cache/cache_session.c

@@ -290,7 +290,7 @@ HTC_RxStuff(struct http_conn *htc, htc_complete_f *func,
 			WRONG("htc_status_e");
 
 		tmo = tn - now;
-		if (!isnan(ti) && ti < tn)
+		if (!isnan(ti) && ti < tn && hs == HTC_S_EMPTY)
 			tmo = ti - now;
 		z = maxbytes - (htc->rxbuf_e - htc->rxbuf_b);
 		assert(z >= 0);