QWbmpHandler: fix integer overflow

Both 'hdr.height' and 'hdr.width' are quint32s. Multiplying them still gives
a quint32, but the lhs expected a qint64.

Fix by casting one of the operands to qint64.

Coverity-Id: 22179
Change-Id: If7385fb42bf994d87ac4e603fa85be4a30ad6d5c
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>