fib: fix crash when create vxlan/vxlan-gpe/geneve/gtpu tunnel.

fib: fix crash when create vxlan/vxlan-gpe/geneve/gtpu tunnel.

Fix vpp crash when create vxlan/vxlan-gpe/geneve/gtpu tunnel
with 0.0.0.0 dst ip in debug build. The ASSERT should be move
out of fib_prefix_from_ip46_addr, which may be called when
create vxlan/vxlan-gpe/geneve/gtpu tunnel with 0.0.0.0 dst ip.

How to reproduce:
1. build debug vpp and run vpp
2. create vxlan t src 192.168.0.2 dst 0.0.0.0 vni 1 instance 1
   create vxlan-gpe tunnel local 192.168.0.2 remote 0.0.0.0 vni 1
   create geneve tunnel local 192.168.0.2 remote 0.0.0.0 vni 1
   create gtpu tunnel src 192.168.0.2 dst 0.0.0.0 teid 1

Type: fix

Change-Id: I19972f6af588f4ff7fd17de1b16b9301e43d596f
Signed-off-by: Huawei LI <lihuawei_zzu@163.com>