Ticket #584 - Existence of an entry is not checked when its password is to be deleted
    
    Bug description: When attempting to delete a password from an
    entry, a password syntax checking api check_pw_syntax_ext missed
    a check if the target entry exists or not.  Note: add and replace
    checks it and handles the case correctly.
    
    Fix description: In this patch the check is added to the delete
    case, as well.
    
    Reviewed by Rich (Thank you!!)
    (cherry picked from commit d559d4665b18702b51161a25737b62799d8ef430)