AUDIO: Fix MSVC warnings

Fixes warning C4245: signed/unsigned mismatch
(seems in line with other decoders)

audio/decoders/adpcm_intern.h

@@ -68,7 +68,7 @@ class ADPCMStream : public SeekableAudioStream {
 
 	virtual bool rewind();
 	virtual bool seek(const Timestamp &where) { return false; }
-	virtual Timestamp getLength() const { return -1; }
+	virtual Timestamp getLength() const { return Timestamp(); }
 
 	/**
 	 * This table is used by some ADPCM variants (IMA and OKI) to adjust the