Library: don't invalidate collection view layout if not running on iPad

Sources/VLCPlaylistViewController.m

@@ -557,7 +557,9 @@
 
 - (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
     [super willRotateToInterfaceOrientation:toInterfaceOrientation duration:duration];
-    [self.collectionView.collectionViewLayout invalidateLayout];
+
+    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
+        [self.collectionView.collectionViewLayout invalidateLayout];
 }
 
 @end