Avoid a notice in `wp_save_image()` by checking `$meta['sizes']['thumbnail']` for `! empty()` before setting a variable to it.

Props MikeHansenMe.
Fixes #17736.



git-svn-id: https://develop.svn.wordpress.org/trunk@28366 602fd350-edb4-49c9-b593-d223f7449a82