Core: Fix potential crashes with mAVStream

src/platform/3ds/main.c

@@ -557,6 +557,7 @@ int main() {
 	rotation.d.readTiltY = _readTiltY;
 	rotation.d.readGyroZ = _readGyroZ;
 
+	stream.videoDimensionsChanged = 0;
 	stream.postVideoFrame = 0;
 	stream.postAudioFrame = 0;
 	stream.postAudioBuffer = _postAudioBuffer;

src/platform/libretro/libretro.c

@@ -197,6 +197,7 @@ void retro_init(void) {
 	logger.log = GBARetroLog;
 	mLogSetDefaultLogger(&logger);
 
+	stream.videoDimensionsChanged = 0;
 	stream.postAudioFrame = 0;
 	stream.postAudioBuffer = _postAudioBuffer;
 	stream.postVideoFrame = 0;