Protect values against integer overflow (#743)

When the code multiplies integer values in an rvalue context before
it's stored in a larger type, the on-the-fly math is stored as int.
The value can overflow before it reaches the wider memory space.

To prevent this, we explicitly cast the result of the arithmetic
to the destination type.

Issues flagged by GitHub CodeQL.

src/FFmpegReader.cpp

@@ -1535,7 +1535,13 @@ void FFmpegReader::ProcessAudioPacket(int64_t requested_frame, int64_t target_fr
 							 audio_frame->nb_samples);       // number of input samples to convert
 
 	// Copy audio samples over original samples
-	memcpy(audio_buf, audio_converted->data[0], audio_converted->nb_samples * av_get_bytes_per_sample(AV_SAMPLE_FMT_S16) * info.channels);
+	memcpy(audio_buf,
+	       audio_converted->data[0],
+	       static_cast<size_t>(
+	           audio_converted->nb_samples
+	           * av_get_bytes_per_sample(AV_SAMPLE_FMT_S16)
+	           * info.channels)
+	      );
 
 	// Deallocate resample buffer
 	SWR_CLOSE(avr);