Assertion due to premature destroy of dialog when media channel init fails during incoming call

[ming]

Scenario (thanks to Fredrik Hansson for the report):

1. In pjsua_call_on_incoming() function, after pjsua_media_channel_init() call returns PJ_SUCCESS, on_incoming_call_med_tp_complete() is called.
2. on_incoming_call_med_tp_complete() returns !PJ_SUCCESS
3. pjsip_dlg_respond() is entered.
4. The response is created without problem and the pjsip_dlg_send_response() is called.
5. We end up on the call to pjsip_dlg_dec_lock_().
6. In the pjsip_dlg_dec_lock() function the session_count == 0 and tsx_count == 0 and we call the unregister_and_destroy_dialog() function.
7. Then the destroy_dialog() is called which destroys the dlg->mutex_ if it exists.
8. After the mutex is destroyed we returns all the way back from the pjsip_dlg_respond() call.
9. After returning from pjsip_dlg_respond() the pjsip_inv_terminate() function is called.
10. In the pjsip_inv_terminate() function the dialog is locked which will lead to an assert since the mutex is already destroyed

[ming]

In 5096:

Fixed #1852: Assertion due to premature destroy of dialog when media channel init fails during incoming call