player/thread: wait for the first chunk in CheckCrossFade()

This fixes MixRamp problems which occur because CheckCrossFade() is
called before the decoder has parsed MixRamp tags after
DecoderClient::Ready().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/2084

NEWS

@@ -42,6 +42,7 @@ ver 0.24 (not yet released)
   - opus: implement bitrate calculation
   - sidplay: require libsidplayfp (drop support for the original sidplay)
   - wavpack: require libwavpack version 5
+  - fix MixRamp bug
 * resampler
   - soxr: require libsoxr 0.1.2 or later
 * player

src/player/Thread.cxx

@@ -857,9 +857,12 @@ Player::CheckCrossFade() noexcept
 		return;
 	}
 
-	if (!IsDecoderAtNextSong() || dc.IsStarting())
+	if (!IsDecoderAtNextSong() || dc.IsStarting() || dc.pipe->IsEmpty())
 		/* we need information about the next song before we
 		   can decide */
+		/* the "pipe.empty" check is here so we wait for all
+                   (ReplayGain/MixRamp) metadata to appear, which some
+                   decoders parse only after reporting readiness */
 		return;
 
 	if (!pc.cross_fade.CanCrossFade(pc.total_time, dc.total_time,