Fix edge case in cast between fn with varargs

* Prevent the next_param_index to become greater than the param_count
  one as it's expected by every other function.
* Fix a typo in a error message.

Closes #4381
21932a0ef2LemonBoy committed on 2/5/2020, 7:31:18 PMparentb022db1
3 files changedLine totals unavailable