fix text scroll speed (#1057)

SourceX/soundsample.cpp

@@ -104,7 +104,9 @@ int SoundSample::GetLength()
 		bytePerSample = 1;
 	}
 
-	return chunk->alen * 1000 / (frequency * channels * bytePerSample);
+	Uint64 ms = 1000; // milliseconds, 64bit to avoid overflow when multiplied by alen
+	Uint32 bps = frequency * channels * bytePerSample; // bytes per second
+	return (Uint32)(chunk->alen * ms / bps);
 };
 
 } // namespace dvl