UW can return a slightly different format for the LSUB when you step outside

UW can return a slightly different format for the LSUB when you step outside
the default_folder_prefix area, also they return NIL for the delimiter which
means the previous regex would always fail as it is looking for a 1 char
delimiter between ""

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5046 7612ce4b-ef26-0410-bec9-ea0150e637f0