codecs: h265: Do not free slice header before using it

codecs: h265: Do not free slice header before using it

The v4l2codecs H.265 decoder uses the
GstH265SliceHdr::entry_point_offset_minus1 array so make sure that it is not
freed before decoding the frame.

Before this patch, some H.265 input would segfault in
gst_v4l2_codec_h265_dec_fill_slice_params() when executing the line:

guint32 entry_point_offset = slice_hdr->entry_point_offset_minus1[i] + 1;

Make sure that the array is not freed before using it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5503>