WPDB: Check that `$wpdb->last_result` is countable before counting with it.

`wpdb::get_col()` iterates over `$wpdb->last_result`, which can be a non-countable value, should the preceding query have failed.

Props spacedmonkey, desrosj, pento.

Merges [43934] into trunk.

See #45299.
Built from https://develop.svn.wordpress.org/trunk@44272


git-svn-id: https://core.svn.wordpress.org/trunk@44102 1a063a9b-81f0-0310-95a4-ce76da25c4cd