codecparsers: h264: fix parsing of slice groups for map type = 2.

When parsing slice groups information for slice_group_map_type = 2, we
should only be reading up to num_slice_groups_minus1 groups since there
is always a "leftover" slice group and as many "foreground" slice groups
as needed.

This fixes parsing for SVCBMT-5 and SVCBMT-12 whereby the base layer would
have incorrectly been parsed to have up to 38 reference frames in list0,
which is not possible.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>