hqx: correct type and size check of info_offset

It is used as size argument of ff_canopus_parse_info_tag, which uses it
as size argument to bytestream2_init, which only supports sizes up to
INT_MAX.
Changing it's type to unsigned simplifies the check.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>