fix: negative line number

[bruno-garcia]

The API docs state 0 if no line is available but on Unity's Mono it can return -1:

Sentry errors with:

    {
      "type": "invalid_data",
      "name": "exception.values.0.stacktrace.frames.2.lineno",
      "value": -1,
      "reason": "expected an unsigned integer"
    }