player/Thread: reset the `decoder_starting` flag in StopDecoder()

Fixes crash bug (assertion failure) if a new "play" command is handled
while the player thread waits for decoder startup.

src/player/Thread.cxx

@@ -400,6 +400,8 @@ Player::StopDecoder(std::unique_lock<Mutex> &lock) noexcept
 		   pipe */
 		ResetCrossFade();
 	}
+
+	decoder_starting = false;
 }
 
 bool