BOutlineListView: fixed expansion causing items to disappear

On a multi-level list view, when expanding an item containing other collapsed lists, the last items disappear due to a wrong element counting.

Change-Id: I2313ad8efb23e8db54cd3563687bf0c2a775a12a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6259
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Automation <[email protected]>
Reviewed-by: Stefano Ceccherini <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

src/kits/interface/OutlineListView.cpp

@@ -846,8 +846,10 @@ BOutlineListView::ExpandOrCollapse(BListItem* item, bool expand)
 				uint32 subLevel = item->fLevel;
 				items++;
 
-				while (--count > 0 && items[0]->fLevel > subLevel)
+				while (count > 0 && items[0]->fLevel > subLevel) {
 					items++;
+					count--;
+				}
 			} else
 				items++;
 		}