Fix an assert in call.c when checking an invalid argument for autorecover

[chalcolith]

The following code crashes the compiler, because {(ISize)} iso^ is an invalid type, and when we check its arg for autorecovery, there is no type to check:

class val Info
  let fn: {(ISize)} iso

  new val create(fn': {(ISize)} iso^) =>
    fn = fn'

  fun call(n: ISize) =>
    fn(n)

actor Main
  new create(env: Env) =>
    let info = Info({(n: ISize) => env.out.print("n = " + n.string()) })
    info.call(123)

This PR adds a null check so we won't assert on the invalid parameter type.

[ponylang-main]

Hi @kulibali,

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 4278.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

Is there an open issue this fixes?

[SeanTAllen]

@kulibali is there an associated issue for this that needs to be closed?

[chalcolith]

No, there's no issue.