wp_category_checklist may output incorrect results

[hailin]

wp_category_checklist has a loop through $categories, it assumes the index begins with 0, and auto-increments. Most $categories will be that case. However, this is a shaky assumption. PHP doesn't guarantee that. I've seem its index begin with 2, for example. and it may not be consecutive.

This will cause the function to output wrong data set sometimes.
The idea is similar to #9661

The fix is to get array_keys first, then loop through the keys. I've verified that the following patch works correctly.

patch

This is a safe fix too, as the logic is obvious.
However when the problem occurs, it could be very difficult to track it down.

the patch is taken against today's WP trunk.

(In [11130]) Allow for non-contiguous array indices. Props hailin. fixes #9670

Ack. I had written a patch in that very same area that fixes a bug in another ticket, related to category selector weirdness.

@ryan: If you find it before I do, please bump it with a "patch is stale" or something, so I get an email.