interface: fix vnet_sw_interface_update_unnumbered

interface: fix vnet_sw_interface_update_unnumbered

Unless a software interface is actually unnumbered, do not set
ip[46]_main.lookup_main.if_address_pool_index_by_sw_if_index [sw_if_index]
to ~0

Fixes this scenario:

loop create
set int state loop0 up
create sub-interface loop0 1
set interface ip addr loop0.1 192.168.1.1/24
delete sub-interface loop0.1
set int ip addr loop0 192.168.1.1/24

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I46141d862fa57d70b93d7bb0c105403708165264
(cherry picked from commit 64d20e76b9108c9158b2b538cd2312d740f48103)