LibGUI: Fix invalid ModelIndices during shift-click multiselection

Previously, If the widget was unfocused, the selection start index
would be invalid. This would result in invalid selections when doing
shift+click on the widget (while it is unfocused).

Now, we reassign the selection start index to current index
before we initiate multiselection, if selection start index is invalid.

Should Fix #11999 and the same bug inside FileManager.

Author: iyush

Userland/Libraries/LibGUI/AbstractView.cpp

@@ -493,6 +493,8 @@ void AbstractView::set_cursor(ModelIndex index, SelectionUpdate selection_update
             if (!m_selection.contains(index))
                 clear_selection();
         } else if (selection_update == SelectionUpdate::Shift) {
+            if (!selection_start_index().is_valid())
+                set_selection_start_index(index);
             select_range(index);
         }