BUG/MINOR: session: ensure conn owner is set after insert into session

BUG/MINOR: session: ensure conn owner is set after insert into session

A crash could occured if a session_add_conn() would temporarily failed
when called via h2_detach(). In this case, connection owner is reset to
NULL. However, if this wasn't the last connection stream, the connection
won't be destroyed. When h2_detach() is recalled for another stream and
this time session_add_conn() succeeds, a crash will occur due to
session_check_idle_conn() invocation with a NULL connection owner.

To fix this, ensure connection owner is always set after
session_add_conn() success.

This bug is considered as minor as the only failure reason for
session_add_conn() is a pool allocation issue.

This should be backported up to all stable releases.