QHash::operator[] could grow the hash even if the key being
looked up already existed. This in turn invalidated all iterators.
Avoid this by refactoring findOrInsert() to not grow if the key
already exists.
Added advantage is that this should make lookups of existing keys
slightly faster.
Fixes: QTBUG-97752
Pick-to: 6.3 6.2
Change-Id: I9df30459797b42c434ba0ee299fd1d55af8d2313
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
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