AUDIO: Fix audio corruption in MS ADPCM decoder

Since _decodedSamples[] is filled with either 2 or 4 samples, we
can't use 1 - (count - 1) to "ensure that acts as a FIFO". When
we have 4 samples, that index will become negative, putting
uninitialized data into the buffer.

We could still use a similar trick, but I think it's much clearer
to use an index variable like this. We need an addition variable
either way.

audio/decoders/adpcm.cpp

@@ -320,10 +320,11 @@ int MS_ADPCMStream::readBuffer(int16 *buffer, const int numSamples) {
 				_decodedSamples[_decodedSampleCount++] = decodeMS(&_status.ch[0], (data >> 4) & 0x0f);
 				_decodedSamples[_decodedSampleCount++] = decodeMS(&_status.ch[_channels - 1], data & 0x0f);
 			}
+			_decodedSampleIndex = 0;
 		}
 
-		// (1 - (count - 1)) ensures that _decodedSamples acts as a FIFO of depth 2
-		buffer[samples] = _decodedSamples[1 - (_decodedSampleCount - 1)];
+		// _decodedSamples acts as a FIFO of depth 2 or 4;
+		buffer[samples] = _decodedSamples[_decodedSampleIndex++];
 		_decodedSampleCount--;
 	}
 

audio/decoders/adpcm_intern.h

@@ -209,6 +209,7 @@ class MS_ADPCMStream : public ADPCMStream {
 			error("MS_ADPCMStream(): blockAlign isn't specified for MS ADPCM");
 		memset(&_status, 0, sizeof(_status));
 		_decodedSampleCount = 0;
+		_decodedSampleIndex = 0;
 	}
 
 	virtual bool endOfData() const { return (_stream->eos() || _stream->pos() >= _endpos) && (_decodedSampleCount == 0); }
@@ -220,6 +221,7 @@ class MS_ADPCMStream : public ADPCMStream {
 
 private:
 	uint8 _decodedSampleCount;
+	uint8 _decodedSampleIndex;
 	int16 _decodedSamples[4];
 };