[nbrmgrd] wrong parsing logic for IPv6 address

[vadymhlushko-mlnx]

Description

Changes were introduced by sonic-swss/pull/1506.
In nbrmgr.cpp Line: 216 a function tokenize has a wrong parsing logic for IPv6 address from APP_DB, because the delimiter for IPv6 (:) is the same as the delimiter for keys from APP_DB.

Steps to reproduce the issue:

1. Create VLAN
2. Assing IPv6 IP address to VLAN (for example: 2000:1::1/64)
3. Add the port to the VLAN
4. Send some traffic to the port from step 3 in order to learn the MAC address.
5. Put port to the DOWN state

Describe the results you received:

swss service crashed with the next logs:
ERR swss#nbrmgrd: :- main: Runtime error: Error converting 2000 to IP address
2000 is the first octet of the assigned IPv6 address

show techsupport:

sonic_dump_r-qa-sw-eth-20162_20210309_143539.tar.gz

[prsunny]

Thanks @vadymhlushko-mlnx for finding this. The fix should be simple similar to this. Would you help fixing?