Fabio Margarido reported that the BYE is not sent upon certain error
conditions. This was because state machine does not allow sending duplicate
BYE in terminating state. Usually, the call state machine switches to
terminating state upon sending BYE.  However, certain error conditions (like
not receiving ACK) causes the session to terminate, and the session state is
change to terminating state outside BYE.

The fix here changes the behaviour of internally generated BYE requests,
they do not change the session state to terminating nor they check if the
session state is already terminating.

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