ignore CANCEL request after having already received final response to INVITE

RFC 3261 section 9 states that a CANCEL has no effect on a
request to a UAS that has already given a final response.  This
patch checks to make sure there is a pending invite before
allowing a CANCEL request to be processed, otherwise it responds
to the CANCEL with a "481 Call/Transaction Does Not Exist".

Review: https://reviewboard.asterisk.org/r/697/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@271977 65c4cc65-6c06-0410-ace0-fbb531ad65f3