Use nullptr over 0 in a few places

Author: arch1t3cht

src/audio_player_portaudio.cpp

@@ -263,7 +263,7 @@ wxArrayString PortAudioPlayer::GetOutputDevices() {
 	list.push_back("Default");
 
 	try {
-		PortAudioPlayer player(0);
+		PortAudioPlayer player(nullptr);
 
 		for (auto it = player.devices.begin(); it != player.devices.end(); ++it)
 			list.push_back(to_wx(it->first));

src/auto4_lua_assfile.cpp

@@ -298,7 +298,7 @@ namespace Automation4 {
 			sty->UpdateData();
 		}
 		else if (lclass == "dialogue") {
-			assert(ass != 0); // since we need AssFile::AddExtradata
+			assert(ass != nullptr); // since we need AssFile::AddExtradata
 			auto dia = new AssDialogue;
 			result.reset(dia);