Fix invalid C lint output

[chvp]

For some bad student code the linter was returning a column 0, which resulted in a negative column in the actual judge output, which is not allowed by Dodona. This created internal errors.

I applied a band-aid fix for this to just set to column to None in this case, since I'd guess that the - 1 actually means that the columns are 1-indexed and 0 means that there isn't really a column to latch on to, but I'm not a 100% sure of that.

[bmesuere]

In Java and Kotlin, we also do -1 so the same fix should be applied there.

[Copilot]

Pull request overview

This PR fixes an issue where the C linter (cppcheck) could return column 0 for certain syntax errors, which after subtracting 1 would result in a negative column value (-1). This violated Dodona's schema requirements causing internal errors. The fix sets the column to None when the computed value would be negative.

Changes:

• Added a safety check in the C linter to set column to None when column 0 is received from cppcheck
• Added a test case with intentionally malformed C code to verify the fix works correctly
• Created a test fixture file with a missing semicolon to reproduce the column 0 scenario

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tested/languages/c/linter.py	Added check to set column to None when value is negative after 1-based to 0-based conversion
tests/test_linters.py	Added new test function to verify linter handles bad column values correctly
tests/exercises/echo-function/solution/bad-column-cppcheck.c	Created test fixture with missing semicolon to trigger column 0 from cppcheck

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[niknetniko]

Makes sense to me. In a follow-up I'll create some helpers to add some more tests for our linters, hopefully preventing similar issues in the future.