Fix String.f32 and String.f64 errors with non null terminated strings

[ergl]

These methods rely on strof/strod, which require null-terminated strings. We
were passing our pointer to C without first checking if our string was
null-terminated, which means that C would be able to read past our memory
allocation.

This fixes #1445

[SeanTAllen]

Is there any way to test this? I can't think of anything but I wanted to bring it up as a topic.

[ergl]

Marking as do not merge for now since the implementation in this PR has a bug related to reallocation. Will fix it soon-ish.

@SeanTAllen It requires some thinking. It is possible, but it requires control over how memory is allocated in order to produce a valid buffer overflow from the C side.

[ergl]

I've added a test and dropped the "We haven't seen errors from this so far, but let's err on the side of caution" from the commit message. Here's a small program that shows the problem with not passing null-terminated strings to C:

actor Main
  new create(env: Env) =>
    (let str1, let str2: String ref) = "1.1".clone().chop(1)
    try
      // Comment the line below to make str1.f32 fail,
      // which means that str1.f32 is looking at the memory from str2
      str2(0)? = 0
      env.out.print(str1.f32()?.string()) // should print "1"
    else
      env.err.print("str1.f32() failed")
    end

[SeanTAllen]

This needs release notes

[ponylang-main]

Hi @ergl,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4132.md to match the number of this pull request.

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## Title

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If a breaking change, make sure to include 1 or more
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Thanks.