isisd: fix a memory leak in isis_spftree_clear()

isisd: fix a memory leak in isis_spftree_clear()

isis_spftree_clear() calls:
  - _isis_spftree_del() to partially delete a spftree instance
    without freeing spftree->route_table and
    spftree->route_table_backup.
  - then _isis_spftree_init() that allocates new spftree->route_table
    and spftree->route_table_backup.

As a consequence, the previous table instances are not referenced and
not freed.

Free the route tables before allocating new ones.

Fixes: 860b75b40e ("isisd: calculate flex-algo constraint spf")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>