decoder/Thread: delete the InputStream on error

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

NEWS

@@ -9,6 +9,7 @@ ver 0.19.18 (not yet released)
   - sidplay: allow building with libsidplayfp instead of libsidplay2
 * output
   - shout: recognize setting "encoder" instead of "encoding"
+* fix memory leak after stream failure
 * fix build failure with Boost 1.61
 * require gcc 4.7 or newer
 

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;