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

props ipm-frommen.
fixes #30095.
Built from https://develop.svn.wordpress.org/trunk@30021


git-svn-id: http://core.svn.wordpress.org/trunk@30021 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove one-time loop and redundant isset() checks.

props ipm-frommen.
fixes #30093.
Built from https://develop.svn.wordpress.org/trunk@30022


git-svn-id: http://core.svn.wordpress.org/trunk@30022 1a063a9b-81f0-0310-95a4-ce76da25c4cd