Fix re-joining a call

[SystemKeeper]

• Regression from Support "already_joined" error message #1358
• Contributes to Connection loss answering call in phone standby #1319

The problem is that we reset the currentRoom when we forceReconnect, which is fine for "normal" forced reconnects:

talk-ios/NextcloudTalk/NCExternalSignalingController.m

Lines 176 to 183 in a09c4a0

	- (void)forceReconnect
	{
	dispatch_async(dispatch_get_main_queue(), ^{
	self->_resumeId = nil;
	self->_currentRoom = nil;
	[self reconnect];
	});
	}

In case our publisher peer fails, we want to rejoin the call, for that, we call forceReconnect on NCExternalSignalingController from NCCallController:

talk-ios/NextcloudTalk/NCCallController.m

Lines 321 to 323 in a09c4a0

	if (self->_externalSignalingController) {
	[self->_externalSignalingController forceReconnect];
	} else {

But for the re-join to work correctly, the NCExternalSignalingController needs to be aware, that we were in a room already:

talk-ios/NextcloudTalk/NCExternalSignalingController.m

Lines 348 to 352 in a09c4a0

	// Re-join if user was in a room
	if (_currentRoom && _sessionChanged) {
	[self.delegate externalSignalingControllerWillRejoinCall:self];
	[[NCRoomsManager sharedInstance] rejoinRoom:_currentRoom];
	}

And this check failed since we reseted currentRoom on a forceReconnect. This results in showing the waiting message "Connecting to the call …" but nothing is ever happening.

How to test

• Establish a call between ios and web
• Hit "Pause" on the Xcode debugger
• Wait until the ios peer is closed/removed from the web
• Continue execution on Xcode