sip: fix pjsip crash if transport send_msg fails

As PJSIP documentation said, pjsip_transport::send_msg must
not call the user callback if PJ_EPENDING is not returned,
like in case of immediate completion or when an error happens.
Was not the case in re-factoring on SipsTransportIce class.
This patch fixes this problem.

Change-Id: I6b8ea8efb6aa61191914d7794f5484612095598a
Tuleap: #106