pjproject: fix crash in pj_turn_session_on_rx_pkt2()

Instead of letting the TURN session destroy itself when finished, the current
code is overriding the user_data attached to the session. This is not needed
because the turn session will be stopped when the timer will complete and
this can cause some crashes in pj_turn_session_on_rx_pkt2 because the data
is erased while not protected via turn_sock->grp_lock() (mostly in turn_sock.c,
on_data_read)

Change-Id: Ia4a12917711b5534b372cb3a26e9141e5393c569
Gitlab: #299