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

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.