res_pjsip: Handle invocation of callback on outgoing request when error occurs.

There are some error cases in PJSIP when sending a request that will
result in the callback for the request being invoked.  The code did not
handle this case and assumed on every error case that the callback was not
invoked.

The code has been changed to check whether the callback has been invoked
and if so to absorb the error and treat it as a success.

ASTERISK-26679
ASTERISK-26699

Change-Id: I563982ba204da5aa1428989a11c06dd9087fea91