BUG/MINOR: prevent conn leak in case of xprt_qmux init failure

BUG/MINOR: prevent conn leak in case of xprt_qmux init failure

In case of XPRT_QMUX init failure on the frontend side, the connection
must immediately be released. This is not the case on the backend side
as a stream can supervize the connection lifetime.

This patch performs the connection free via conn_complete_session(). As
conn is flagged with CO_FL_ERROR, this will automatically fail and
invoke session_kill_embryonic(), which ensures the session and its
connection are both freed as wanted in this case.

No need to backport.