DEBUG: stream-int: Check CS_FL_WANT_ROOM is not set with an empty input buffer

DEBUG: stream-int: Check CS_FL_WANT_ROOM is not set with an empty input buffer

In si_cs_recv(), the mux must never set CS_FL_WANT_ROOM flag on the
conn-stream if the input buffer is empty and nothing was copied. It is
important because, there is nothing the app layer can do in this case to
make some room. If this happens, this will most probably lead to a ping-pong
loop between the mux and the stream.

With this BUG_ON(), it will be easier to spot such bugs.