decoder: h264: initialize PPS's slice_group_id

decoder: h264: initialize PPS's slice_group_id

When the GstVaapiParserInfoH264 is allocated, the memory is not initialized,
so it contains random data.

When gst_h264_parser_parse_pps() fails, the PPS structure keeps slice_group_id
pointer uninitialized, leading to a segmentation fault when the memory is
freed.

This patch prevents this by initializing the slice_group_id before the PPS
parsing.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=754845