bup-restore.md: always initialize root_meta in do_root.

Previously, using bup split with bup restore would cause an error:

  UnboundLocalError: local variable 'root_meta' referenced before assignment

Signed-off-by: itxx00 <itxx00@gmail.com>
[rlb@defaultvalue.org: adjust commit message.  Initialize root_meta
 unconditionally before guard to match other code.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>