Avoid index errors that cause Mypy to raise Internal Error

[jimmylai]

Avoid index errors that cause Mypy to raise Internal Error

Mypy failed by the following internal error in our codebase

error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
If this issue continues with mypy master, please report a bug at https://github.com/python/mypy/issues
version: 0.991

After investigation, it's caused by an IndexError in mypy_django_plugin.transformers.orm_lookups
This fix adds boundary checks on arg_types and arg_names to avoid the IndexError.
With this patch, Mypy runs successfully without raising internal errors.

Related issues

[sobolevn]

Thank you!