Fix a copy/paste issue in Custom_Image_Header::get_header_dimensions() introduced in [27497].

props ipm-frommen.
fixes #30095.

git-svn-id: https://develop.svn.wordpress.org/trunk@30021 602fd350-edb4-49c9-b593-d223f7449a82
Remove one-time loop and redundant isset() checks.

props ipm-frommen.
fixes #30093.

git-svn-id: https://develop.svn.wordpress.org/trunk@30022 602fd350-edb4-49c9-b593-d223f7449a82