combobox: Fix reentrancy in gtk_combo_box_popdown()

combobox: Fix reentrancy in gtk_combo_box_popdown()

If called when already popped down, warnings would be issued due
to priv->grab_pointer being unexpectedly NULL, this would happen
in regular operation when selecting items in appears-as-list mode.

So both add a NULL check for priv->grab_pointer, and bail out early
if the popup window is already hidden.