BUG/MEDIUM: stconn: Do nothing in sc_conn_recv() when the SC needs more room

We erroneously though that an attempt to receive data was not possible if the SC
was waiting for more room in the channel buffer. A BUG_ON() was added to detect
bugs. And in fact, it is possible.

The regression was added in commit 341a5783b ("BUG/MEDIUM: stconn: stop to
enable/disable reads from streams via si_update_rx").

This patch should fix the issue #2115. It must be backported if the commit
above is backported.
diff --git a/src/stconn.c b/src/stconn.c
index b1d02d9..95fe9c5 100644
--- a/src/stconn.c
+++ b/src/stconn.c
@@ -1194,7 +1194,7 @@
 	/* If another call to sc_conn_recv() failed, and we subscribed to
 	 * recv events already, give up now.
 	 */
-	if (sc->wait_event.events & SUB_RETRY_RECV)
+	if ((sc->wait_event.events & SUB_RETRY_RECV) || sc_waiting_room(sc))
 		return 0;
 
 	/* maybe we were called immediately after an asynchronous shutr */
@@ -1220,7 +1220,6 @@
 
 	/* prepare to detect if the mux needs more room */
 	sc_ep_clr(sc, SE_FL_WANT_ROOM);
-	BUG_ON(sc_waiting_room(sc));
 
 	if ((ic->flags & (CF_STREAMER | CF_STREAMER_FAST)) && !co_data(ic) &&
 	    global.tune.idle_timer &&