backend: fix panic when reading result set fails

[djshow832]

What problem does this PR solve?

Issue Number: close #485

Problem Summary:
TiProxy doesn't handle errors when processing result sets, and it reports nil pointer reference.

What is changed and how it works:
Check the error first.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

- Fix panic when a TiDB fails during session migration.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@384e746). Click here to learn what that means.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #486   +/-   ##
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  Coverage        ?   64.33%           
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  Files           ?       61           
  Lines           ?     5787           
  Branches        ?        0           
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  Hits            ?     3723           
  Misses          ?     1776           
  Partials        ?      288           

Flag	Coverage Δ
unit	64.33% <100.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-19 08:43:26.95901315 +0000 UTC m=+1366233.981259536: ☑️ agreed by xhebox.