Revert "player/Thread: remove unnecessary "pipe" check"

This reverts commit ff3e2c0514.  The
check was necessary, after all, because this is what checked whether
the decoder had finished the current or the next song.

> The "queued" flag can only possibly be set if the decoder is still
> decoding the current song or if the decoder is stopped.

That was wrong because ProcessCommand() sets `queued=true` and also
starts the decoder (if it was idle).

> This is also what the following assert() checks.

That was also wrong, because the assert() has two conditions.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/566

NEWS

@@ -1,4 +1,5 @@
 ver 0.21.10 (not yet released)
+* fix crash bug (0.21.9 regression)
 
 ver 0.21.9 (2019/05/20)
 * input

src/player/Thread.cxx

@@ -996,7 +996,7 @@ Player::Run() noexcept
 			}
 		}
 
-		if (dc.IsIdle() && queued) {
+		if (dc.IsIdle() && queued && IsDecoderAtCurrentSong()) {
 			/* the decoder has finished the current song;
 			   make it decode the next song */