sorter: fix panic when getting memory pressure fails (#2014)

[ti-chi-bot]

This is an automated cherry-pick of #2014

What problem does this PR solve?

• When /proc/meminfo fails to open, TiCDC will panic because of inappropriate error handling. This usually happens when the OS does not allow TiCDC any more file descriptors.

What is changed and how it works?

• Fixed error handling when reading system memory pressure.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix panic when TiCDC fails to read /proc/meminfo

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• lonng

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[lonng]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c4c394d

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[amyangfei]

/run-unit-tests