command/fingerprint: fix mutex locking bug

GetChromaprintCommand::DecodeStream() is called without holding the mutex.

src/command/FingerprintCommands.cxx

@@ -107,11 +107,10 @@ GetChromaprintCommand::DecodeStream(InputStream &input_stream,
 
 	/* rewind the stream, so each plugin gets a fresh start */
 	try {
-		input_stream.Rewind();
+		input_stream.LockRewind();
 	} catch (...) {
 	}
 
-	const ScopeUnlock unlock(mutex);
 	plugin.StreamDecode(*this, input_stream);
 }