pixdesc: Only check against valid entries when iterating over lists of enums

Some of these enums have gaps in between their values, since they correspond
to the values in various specs, instead of being an incrementing list.

Fixes segfaults when, for example, using the valid API call:

   av_color_primaries_from_name("jecdec-p22");

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>