Modify our handling of 491 responses to drop any pending reinvite retry scheduler...

Modify our handling of 491 responses to drop any pending reinvite retry scheduler entries if we get a new 491.

This prevents a scheduler entry from leaking if we receive a 491 response when one is pending.  If a scheduler entry leaks, the pvt it is associated my get destroyed before the scheduler entry fires, and then memory corruption and crashes can occur when the scheduled reinvite attempts to access and modify the memory of the destroyed pvt.

ABE-2543

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@294163 65c4cc65-6c06-0410-ace0-fbb531ad65f3