QMultiHash::equal_range crashes when called in a const member function.
The Data `d` is a NULL pointer when calling equal_range()
before inserting data into an empty QMultiHash.
Then calling`d->find` crashes.
Fixes: QTBUG-89687
Pick-to: 6.0
Change-Id: I10c3d196cbc72aed8c8c922ef16534bba51037b7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>