These problems are mostly introduced in 1.12.5:
- assert failed if INVITE was received when calling
  (reported by Michael Jerris)
- assert failed if nua_invite() was called second time before receiving
  final response to first INVITE (ditto)
- ACK is now always sent if session was terminated after receiving 2XX
- if nua has a pending incoming INVITE request, return 491 to
  nua_invite()

darcs-hash:20070214201847-55b16-962b2de246eee8100e4dba8d7a28c507b13c376c.gz