rustls_error can cause undefined behavior when a non-error is passed

[jsha]

Steps to reproduce:

  char buf[256];
  size_t n;
  fprintf(stderr, "watch this:\n");
  rustls_error(0, buf, sizeof(buf), &n);

Result:

watch this:
Segmentation fault (core dumped)

This is because it's undefined behavior in Rust to have an enum where the contents are out of range for the enum definition, and there's no rustls_result corresponding to 0.

To fix this, we need to do some validation any time we accept a rustls_result from C. I think that's just in rustls_error.

[jsha]

Fixed in #195