Fix out-of-bounds access in command-line parser

[lava]

📔 Description

Fix out-of-bounds access in command-line parser

For some errors, the command-line parser returns `last`
as the position of the error which we must not dereference
when building an error message.

This bug was originally found by an automated fuzzing run
via Code Intelligence.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

[dominiklohmann]

I checked this out locally to verify that it fixes the issue.

The CI errors because fptr in the unit tests is unused, but other than that everything runs through locally.