Validate get_post_templates() File Cache

[Howdy_McGee]

The WP_Theme Class method get_files() caches the all the theme's files. Whenever anything calls WP_Theme->get_post_templates() it pulls from the transient but whenever looping through the file it does not verify the cached file exists before calling `file_get_content()`. I suggest we change LNL1048 to the following:

if ( ! ( file_exists( $full_path ) && preg_match( '|Template Name:(.*)$|mi', file_get_contents( $full_path ), $header ) ) ) {
        continue;
}

If a file is removed within the hour cache limit, Pages Post Templates throw out some errors.

[Howdy_McGee]

WP_Theme Post Template Errors

[Clorith]

Duplicate of #42573