QMenu check for null result from QPlatformMenu::menuItemForTag

QMenu needs to check result of QPlatformMenu::menuItemForTag to avoid
a crash dereferencing a null pointer.

Task-number: QTBUG-42327
Change-Id: Ie54a94caec7a5d756c459741df182fbe4e38bec0
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>