fix potential npe in executeTS (#2371)

[ti-srebot]

cherry-pick #2371 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tics repo:
git pr https://github.com/pingcap/tics/pull/2377

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tics.git pr/2377:release-4.0-245dd77cbf91

---

What problem does this PR solve?

Issue Number: close #2342

Problem Summary:
As the issue described.

What is changed and how it works?

Proposal: xxx

What's Changed:
The root cause is using an uninitialized pointer, this pr fix it by not using the pointer until it is initialized propertly.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

Release note

• fix potential npe in executeTS during DAG compile

[JaySon-Huang]

LGTM

[windtalker]

/merge

[ti-srebot]

/run-all-tests