config_format: fix SEGV from missing return check

[DavidKorczynski]

Fixes: https://issues.oss-fuzz.com/issues/42530021

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[DavidKorczynski]

CI failure is not due to this PR, can you please take a look @edsiper ?

[leonardo-albertovich]

I've triggered a CI retry but even if CI keeps failing this is mergeable and should not be delayed.