WPDB: Before fetching results, make sure we have a valid result resource to fetch them from.

Fixes #27982.

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git-svn-id: http://core.svn.wordpress.org/trunk@30667 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve various `@param` docs for `src/wp-admin/includes/class-wp-filesystem-*.php`.

See #30224.

Built from https://develop.svn.wordpress.org/trunk@30678


git-svn-id: http://core.svn.wordpress.org/trunk@30668 1a063a9b-81f0-0310-95a4-ce76da25c4cd