Player: Fix crossfade crash when decoding fails

When the decoding of a track fails, `current_item_` is set to an invalid address, if the Crossfade option is enabled, the `Player::TrackAboutToEnd` method does not check whether `current_item_` is a valid pointer or not, causing a segmentation fault.

Player: Removed extra space

src/core/player.cpp

@@ -867,6 +867,9 @@ void Player::TrackAboutToEnd() {
   if (engine_->is_autocrossfade_enabled()) {
     // Crossfade is on, so just start playing the next track.  The current one will fade out, and the new one will fade in
 
+    // If the decoding failed, current_item_ will be null
+    if (!current_item_) return;
+
     // But, if there's no next track, and we don't want to fade out, then do nothing and just let the track finish to completion.
     if (!engine_->is_fadeout_enabled() && !has_next_row) return;