Backports

[mhugo]

Description

Backports from master:

• fix $length in labels
• avoid an undefined behaviour

Checklist

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• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
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• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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[nyalldawson]

+1 . Can you include 1b228bb also?

[mhugo]

@nyalldawson Sure, and thanks for the addition