Previously, when deciding where the actual data is, Frame was calling
padding() to test if offset is needed. A curious case with a DATA frame
containing compressed body and having 'PADDED' flag set with a padding
equal to ... 0, ended in a decompression error (and assert in 6.2 code).
Pick-to: 6.2 5.15
Fixes: QTBUG-97179
Change-Id: I9341a4d68510aa4c26f4972afdcd09a530d5a367
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>