Fix a panic by gnmi subscribe to DB change with invalid xpath

[wumiaont]

Why I did it

It is found that if I run gnmi subscribe to db change with an invalid xpath, gnmi server has a panic because of nil pointer dereference.

Test is against multi-asic chassis. In telemtery test test_on_change_updates is did not handle the multi-asic properly which sends out gnmi subscribe like this: python /root/gnxi/gnmi_cli_py/py_gnmicli.py -g -t 10.250.6.233 -p 50052 -m subscribe -x NEIGH_STATE_TABLE -xt STATE_DB -o ndastreamingservertest --subscribe_mode 0 --submode 1 --interval 0 --update_count 2 --create_connections 1

gnmi container restarts with a panic.

Correct CLI commands should be: python /root/gnxi/gnmi_cli_py/py_gnmicli.py -g -t 10.250.6.233 -p 50052 -m subscribe -x NEIGH_STATE_TABLE/asic0 -xt STATE_DB -o ndastreamingservertest --subscribe_mode 0 --submode 1 --interval 0 --update_count 2 --create_connections 1

How I did it

Fix the deference of nil pointer with this scenario.

How to verify it

With modified code, the above CLI will not cause gnmi panic. The connection is properly closed.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202405

[zbud-msft]

Introduced by #267

[arlakshm]

/Azp run sonic-net.sonic-gnmi

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[bingwang-ms]

@wumiaont Please raise a PR to 202405 branch to address cherry-pick conflict

[wumiaont]

#267 which introduced this issue was not merged to 202405 so no need to put this fix into 202405. Please remove the "[Request for 202405 Branch]" label

[wumiaont]

@bingwang-ms @zbud-msft Please help to remove the Request for 202405 Branch label as issues was not in 202405.