When generating updates for a peer that has 'rde evaluate all' set the

When generating updates for a peer that has 'rde evaluate all' set the
old prefix pointer is most probably NULL. If a secondary route is removed
the withdraw would not happen because old == NULL which skips the withdraw.

Access to old is only needed to extract the prefix. So instead extract the
prefix early and use it for both cases. So if 'rde evaluate all' is used
the code tries all prefixes and if none is allowed a withdraw is issued.

Problem noticed and fix tested by Pier Carlo Chiodi