BUG/MEDIUM: mux-fcgi: Return from detach if server don't keep the connection

When the last stream is detached from a FCGI connection, if the server don't add
the connection in its idle list, the connection is destroyed. Thus it is
important to exist immediately from the detach function. A return statement is
missing here.

This bug was introduced in the commit 2444aa5b6 ("MEDIUM: sessions: Don't be
responsible for connections anymore.").

It is a 2.2-dev bug. No need to backport.

src/mux_fcgi.c

@@ -3542,6 +3542,7 @@ static void fcgi_detach(struct conn_stream *cs)
 					/* let's kill the connection right away */
 					fconn->conn->mux->destroy(fconn);
 					TRACE_DEVEL("outgoing connection killed", FCGI_EV_STRM_END|FCGI_EV_FCONN_ERR);
+					return;
 				}
 			}
 		}