Fix several esoteric errors related to AJAX unit tests for comments:

* `wp_ajax_get_comments()` relies on the `$post_id` global - even though `$_POST['p']` is passed to every action in the test methods. If `$post_id` is still lingering in between tests and doesn't match `p` in the request, the cap check might pass while the queries for comments will blow up. I added `unset( $GLOBALS['post_id'] )` to `Tests_Ajax_GetComments::setUp()`.
* If the global `$post_id` is empty, but `$_REQUEST['p']` is not, `$post_id` is now set to `absint( $_REQUEST['p'] )` and sanity-checked in `wp_ajax_get_comments()`.
* `map_meta_cap()` always assumes that `get_comment()` succeeds when checking for the `edit_comment` cap. It doesn't. I added sanity checks in a few places where it will break early if `get_post()` or `get_comment()` are empty.
* `wp_update_comment()` always assumes `get_comment()` succeeds. It doesn't. I added a check for empty.

All AJAX unit tests run and pass in debug mode. All general unit tests pass against these changes.

Fixes #25282.


Built from https://develop.svn.wordpress.org/trunk@25438


git-svn-id: http://core.svn.wordpress.org/trunk@25360 1a063a9b-81f0-0310-95a4-ce76da25c4cd