macosx: Don't yield an item that may be NULL.

This should fix a bunch of bug report we had from apple-bureport mailing-list.

modules/gui/macosx/playlistinfo.m

@@ -427,7 +427,7 @@ error:
 
 - (input_item_t *)item
 {
-    vlc_gc_incref( p_item );
+    if( p_item ) vlc_gc_incref( p_item );
     return p_item;
 }