nat: fix static mapping segv

nat: fix static mapping segv

Adding a nat44 static mapping during startup on a DHCP interface leads
to a segv via this path:
- dhcp_client_acquire_address
- ip4_add_del_interface_address
- ip4_add_del_interface_address_internal
- nat44_ed_add_del_interface_address_cb
- nat44_ed_add_static_mapping
- ip4_interface_first_address

Type: fix

Signed-off-by: Joshua Roys <roysjosh@gmail.com>
Change-Id: I38dac8a096b052550f2b87b4e13a950d2cd868b0
(cherry picked from commit f9074fc51544d1849a2df06713c1ccf6cb2eb908)