res_pjsip_session.c: Check for port of zero on incoming SDP.

res_pjsip_session.c: Check for port of zero on incoming SDP.

If a re-invite comes in initiating T.38, but there is no c line in the
SDP and the port is also 0, a crash can occur. A check is now done on
the port to see if the steam is already declined, preventing the crash.
The logic was moved to res_pjsip_session.c because it is handled in a
similar manner in later versions of Asterisk.

ASTERISK-28612
Reported by: Salah Ahmed

Change-Id: Ifc4a0d05b32c7f2156e77fc8435a6ecaa6abada0