popupMenu.js: avoid unintended side effect of unexpected recursive call of PopupMenu.close

Calling this._boxPointer.hide in PopupMenu.close triggers
PopupMenuManager._onKeyFocusChanged,
which ends up calling PopupMenu.close recursively, causing
global.menuStackLength to be decremented twice.
Merge pull request #2048 from autarkper/popupmenu-recursive-close-fix

popupMenu.js: avoid unintended side effect of unexpected recursive call of PopupMenu.close