On windows host using ftp, the 'islink' key is not available

[joostdekeijzer]

WP_Filesystem_FTPext::parselisting creates the $b array. Almost near the end of the method the value of $islink? is tested but that key isn't set on windows servers.

https://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/class-wp-filesystem-ftpext.php#L356

Either set the islink key (to false) on windows servers or move the islink test to the "not windows" part of the if-elseif.

[jesin]

Check if index islink is set before evaluating it

[jesin]

Calling WP_Filesystem_FTPext::parselisting( $line ) on Windows displays the following notice.

Notice: Undefined index: islink in /var/www/wp-admin/includes/class-wp-filesystem-ftpext.php on line 357

The patch checks if $b['islink'] is set before checking its value.

[jesin]

Update patch for latest revision and add curly braces

[wonderboymusic]

In 35476:

Filesystem: in WP_Filesystem_FTPext::parselisting(), check for the existence of $b['islink'] before using it.

Props jesin.
Fixes #27975.