mxfdec: fix NULL checking in mxf_get_sorted_table_segments()

The following out-of-memory check is broken.

    *sorted_segments  = av_mallocz(...);
    if (!sorted_segments) { ... }

The correct NULL check should use *sorted_segments.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
