decoder/Thread: delete the InputStream on error

Fixes memory leak after stream failure.  See
https://bugs.musicpd.org/view.php?id=4562

src/decoder/DecoderThread.cxx

@@ -98,6 +98,7 @@ decoder_input_stream_open(DecoderControl &dc, const char *uri)
 
 	if (!is->Check(error)) {
 		dc.Unlock();
+		delete is;
 
 		LogError(error);
 		return nullptr;