res_pjsip_session.c: Fix unbound srv failover tests.

res_pjsip_session.c: Fix unbound srv failover tests.

Commit 1b666549f33d69dc080b212bf92126f3bc3a18b2 broke the srv failover
functionality if a TCP connection gets disconnected.  Under these
conditions, session_inv_on_state_changed() gets a
PJSIP_EVENT_TRANSPORT_ERROR and restarts the INVITE transaction on a new
transport.  Unfortunately, session_inv_on_tsx_state_changed() also gets
the same PJSIP_EVENT_TRANSPORT_ERROR event and unconditionally terminates
the session.

* Made session_inv_on_tsx_state_changed() complete terminating the session
on PJSIP_EVENT_TRANSPORT_ERROR only if the session state is still
PJSIP_INV_STATE_DISCONNECTED.

ASTERISK-26305 #close
Reported by: Richard Mudgett

Change-Id: If736e766b5c55b970fa38ca6c8a885caf27b897d