Don't rely on uninitialized data

Don't rely on uninitialized data

HB_GetCharAttributes used to require a zero-initialized array for
attributes, as it selectively sets relevant bits for each character. We
ease that requirement by always initializing the attributes buffer
explicitly with memset.

Task-number: QT-4911
Reviewed-by: Ritt Konstantin
(cherry picked from commit 4cb9db404224c55859713c282aa90409e375c372)