pjsip: avoid edge case potential crash in answer()

When accidentally compiling against a wrong version of
pjsip headers with a different pjsip_inv_session size,
the invite_tsx structure could be null in the answer()
function.  This led to a crash because it attempted to
send the session response with an uninitialized packet
pointer.  This patch presets packet to null and adds a
diagnostic log message to explain why the call fails.

Review: https://reviewboard.asterisk.org/r/3267/
........

Merged revisions 408970 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408971 65c4cc65-6c06-0410-ace0-fbb531ad65f3