FFMPEG Plugin: Refactor out checking of invalid conditions in audio path.

- Main reason for this refactoring is to increase readability and thus make
  audio decode path more comprehensible.

- Added documentation for the new method accordingly.

- Small change in calculating the decoded data size to clear when error occurs
  during decoding. This way it is more readable and more consistent with
  calculations of decoded data size on other locations.

- No functional change intended.

src/add-ons/media/plugins/ffmpeg/AVCodecDecoder.cpp

@@ -672,17 +672,7 @@ AVCodecDecoder::_DecodeNextAudioFrame()
 	fRawDecodedAudio->data[0] = fDecodedData;
 
 	while (fRawDecodedAudio->nb_samples < fOutputFrameCount) {
-		// Check conditions which would hint at broken code below.
+		_CheckAndFixConditionsThatHintAtBrokenAudioCodeBelow();
-		if (fDecodedDataBufferSize < 0) {
-			fprintf(stderr, "Decoding read past the end of the decoded data "
-				"buffer! %ld\n", fDecodedDataBufferSize);
-			fDecodedDataBufferSize = 0;
-		}
-		if (fTempPacket.size < 0) {
-			fprintf(stderr, "Decoding read past the end of the temp packet! "
-				"%d\n", fTempPacket.size);
-			fTempPacket.size = 0;
-		}
 
 		bool decodedDataBufferHasData = fDecodedDataBufferSize > 0;
 		if (decodedDataBufferHasData) {
@@ -700,7 +690,9 @@ AVCodecDecoder::_DecodeNextAudioFrame()
 		if (decodingStatus != B_OK) {
 			// Assume the audio decoded until now is broken so replace it with
 			// some silence.
-			memset(fDecodedData, 0, fRawDecodedAudio->data[0] - fDecodedData);
+			size_t decodedDataSize
+				= fRawDecodedAudio->nb_samples * fOutputFrameSize;
+			memset(fDecodedData, 0, decodedDataSize);
 
 			if (!fAudioDecodeError) {
 				// Report failure if not done already
@@ -730,6 +722,28 @@ AVCodecDecoder::_DecodeNextAudioFrame()
 }
 
 
+/*!	\brief Checks fDecodedDataBufferSize and fTempPacket for invalid values,
+		reports them and assigns valid values.
+
+	Note: This method is intended to be called before any code is executed that
+	deals with moving, loading or decoding any audio frames.
+*/
+void
+AVCodecDecoder::_CheckAndFixConditionsThatHintAtBrokenAudioCodeBelow()
+{
+	if (fDecodedDataBufferSize < 0) {
+		fprintf(stderr, "Decoding read past the end of the decoded data "
+			"buffer! %ld\n", fDecodedDataBufferSize);
+		fDecodedDataBufferSize = 0;
+	}
+	if (fTempPacket.size < 0) {
+		fprintf(stderr, "Decoding read past the end of the temp packet! %d\n",
+			fTempPacket.size);
+		fTempPacket.size = 0;
+	}
+}
+
+
 /*!	\brief Moves audio frames from fDecodedDataBuffer to fRawDecodedAudio (and
 		thus to fDecodedData) and updates the start times of fRawDecodedAudio,
 		fDecodedDataBuffer and fTempPacket accordingly.

src/add-ons/media/plugins/ffmpeg/AVCodecDecoder.h

@@ -61,6 +61,7 @@ private:
 							media_header* mediaHeader,
 							media_decode_info* info);
 			status_t	_DecodeNextAudioFrame();
+			void		_CheckAndFixConditionsThatHintAtBrokenAudioCodeBelow();
 			void		_MoveAudioFramesToRawDecodedAudioAndUpdateStartTimes();
 			status_t	_LoadNextAudioChunkIfNeededAndAssignStartTime();
 			status_t	_DecodeSomeAudioFramesIntoEmptyDecodedDataBuffer();