Fixed a memory leak in the dummy video provider detected by valgrind. The video frame was being allocated but never deallocated.

Originally committed to SVN as r2962.

aegisub/src/video_provider_dummy.cpp

@@ -194,6 +194,7 @@ DummyVideoProvider::DummyVideoProvider(double _fps, int frames, int _width, int
 //////////////
 // Destructor
 DummyVideoProvider::~DummyVideoProvider() {
+	frame.Clear();
 }