Fix c curl generator

[amoweb]

Fix for issue #21092

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• [ x Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

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  ./bin/utils/export_docs_generators.sh || exit
  

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@eafer

[eafer]

Hi! I just took a quick look at the new generated samples, and I see that petId for deletePet becomes a pointer. What's the point of that? The petId is not nullable, right?

[eafer]

Also, would you mind adding some tests for these issues to the petstore.yaml for C? That would make it much easier to understand what you are doing, and it would prevent other people from breaking this again in the future.