Add asserts for missing images and sound effects

I don't want to add too many asserts, because sometimes it's okay if a
file is missing (mmmmmm.vvv). But currently, the game basically expects
all images and sound effects to be present. That might change in the
future, but for now, these asserts are okay.

desktop_version/src/GraphicsResources.cpp

@@ -37,6 +37,11 @@ static SDL_Surface* LoadImage(const char *filename, bool noBlend = true, bool no
 	unsigned char *fileIn;
 	size_t length;
 	FILESYSTEM_loadAssetToMemory(filename, &fileIn, &length, false);
+	if (fileIn == NULL)
+	{
+		SDL_assert(0 && "Image file missing!");
+		return NULL;
+	}
 	if (noAlpha)
 	{
 		lodepng_decode24(&data, &width, &height, fileIn, length);

desktop_version/src/SoundSystem.cpp

@@ -38,6 +38,7 @@ SoundTrack::SoundTrack(const char* fileName)
 	if (mem == NULL)
 	{
 		fprintf(stderr, "Unable to load WAV file %s\n", fileName);
+		SDL_assert(0 && "WAV file missing!");
 		return;
 	}
 	SDL_RWops *fileIn = SDL_RWFromConstMem(mem, length);