As QMultiHash uses a pointer for the data, nullptr dereference is a
thing, so check for valid d before doing anything in count()
Fixes: QTBUG-91704
Pick-to: 6.0 6.1
Change-Id: Ia20440cd7bdc03cb09c77f796fb9c5b52765eac5
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>