decoder/Thread: clear the command after catching an exception

If an early exception gets caught (e.g. from
AllocatedPath::FromUTF8Throw()) before
DecoderControl::CommandFinishedLocked() is called, the decoder thread
would go in an endless loop, because DecoderCommand::START is still
set.

Closes #118

src/decoder/DecoderThread.cxx

@@ -508,6 +508,7 @@ try {
 	decoder_run_song(dc, song, uri_utf8, path_fs);
 } catch (...) {
 	dc.state = DecoderState::ERROR;
+	dc.command = DecoderCommand::NONE;
 	dc.error = std::current_exception();
 	dc.client_cond.signal();
 }