media.cpp: prevent AudioMediaPlayer destructor from doubly removing port

[ecolsen7]

The AudioMediaPlayer destructor has an issue where it effectively destroys the player's conference port twice, in a thread-unsafe way that leaves an opportunity for another thread to see a conference port used by a player as free-for-use, and reserve it for other purposes before ultimately being destroyed again, causing crashes.

We see this race condition ~1 in 5 times when attempting to disrupt an ongoing call with a talkdown call, where StopTransmit() is interrupted by the PJSIP thread answering the incoming call.

Here's an example of how this can happen:

AudioMediaPlayer::~AudioMediaPlayer()
{
    if (playerId != PJSUA_INVALID_ID) {
        // Example: media player is using slot 2, aka pjsua_var.player[playerId].slot == 2

        unregisterMediaPort(); <----- calls pjsua_conf_remove_port() which removes slot 2, but does not de-init/clear pjsua_var.player[playerId].slot

        // INCOMING CALL THREAD OCCURS HERE - finds any empty conf port and adds a new port for the incoming call, sees slot 2 is free and sets up port there

        pjsua_player_destroy(playerId); <----- also calls pjsua_conf_remove_port() on pjsua_var.player[playerId].slot, which still == 2
        
        // This removes the now-active talkdown port, program crashes on talkdown connect audio
    }
}

The approach taken in this PR to fix this problem is taken from a pattern used elsewhere in media.cpp presumably to prevent this exact problem - invalidating the AudioMedia id such that unregisterMediaPort() does not attempt to remove the port unnecessarily:

pjproject/pjsip/src/pjsua2/media.cpp

Lines 665 to 670 in 7eec09d

	DevAudioMedia::~DevAudioMedia()
	{
	/* Avoid removing this port (conf port id=0) from conference */
	this->id = PJSUA_INVALID_ID;
	unregisterMediaPort();
	}

Relevant segment of unregisterMediaPort():

pjproject/pjsip/src/pjsua2/media.cpp

Lines 177 to 182 in 7eec09d

	void AudioMedia::unregisterMediaPort()
	{
	if (id != PJSUA_INVALID_ID) {
	pjsua_conf_remove_port(id);
	id = PJSUA_INVALID_ID;
	}

This ensures that the only attempt to remove an AudioMediaPlayer's port occurs in pjsua_player_destroy(), therefore guaranteeing that pjsua_player_destroy() does not delete a now-occupied port mistakenly.