pjsip: avoid edge case potential crash in answer()

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/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@408970 65c4cc65-6c06-0410-ace0-fbb531ad65f3