lb: Fix generating illegal key in per-port vip

VIP prefix index becomes always 0 when adding a VIP which is already registered different port, causing LB config crash.
This change assigns the same VIP prefix index to the same VIP.

Ticket: https://jira.fd.io/browse/VPP-1834
Type: fix

Signed-off-by: Yasuhiro Nakamura <yanakamu@yahoo-corp.jp>
Change-Id: Ib63b3e58db9bd85714d68cd1292aadd0c8580da8
diff --git a/src/plugins/lb/lb.c b/src/plugins/lb/lb.c
index 4dbf134..2fc38e2 100644
--- a/src/plugins/lb/lb.c
+++ b/src/plugins/lb/lb.c
@@ -906,6 +906,8 @@
       if (!lb_vip_port_find_diff_port(&(vip->prefix), vip->plen,
                                       vip->protocol, vip->port, &vip_idx))
         {
+          lb_vip_t *exists_vip = lb_vip_get_by_index(vip_idx);
+          *vip_prefix_index = exists_vip->vip_prefix_index;
           return;
         }