Fix #
3001523: Cancelling a long pending file transfer crashes Pidgin
After some time of inactivity, Communicator sends SIP BYE causing SIPE to deallocate corresponding sip_dialog.
When file transfer was afterwards canceled by user, code attempted to access the freed structure, leading to
segfault.
Terminating dialog now cancels all pending file transfers with that particular contact. Accept/decline screen
window is replaced with a message that remote peer cancelled the transfer, not allowing user to make invalid
action anymore.