cherry-pick #16038 to release-3.0

What problem does this PR solve?

Issue Number: close #16037

Problem Summary:

port probing will establish tcp connection, then exit without send any auth packet, so conn's session ctx should be nil and this cause crash in audit plugin code path.

What is changed and how it works?

What's Changed:

ensure session ctx is not nil before do reject audit.

How it Works:

check nil

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Manual test

start tidb with audit plugin, and telnet 0.0.0.0 4000 then control-c to exit telent

and watch tidb-stderr.log

Side effects

• n/a

Release note

This change is