Fix potential crash when calling an overloaded function (#1327)

* Fix potential crash when calling an overloaded function

The crash would occur if:
- dispatcher() uses two-pass logic (because the target is overloaded and some arguments support conversions)
- the first pass (with conversions disabled) doesn't find any matching overload
- the second pass does find a matching overload, but its return value can't be converted to Python

The code for formatting the error message assumed `it` still pointed to the selected overload,
but during the second-pass loop `it` was nullptr. Fix by setting `it` correctly if a second-pass
call returns a nullptr `handle`. Add a new test that segfaults without this fix.

* Make overload iteration const-correct so we don't have to iterate again on second-pass error

* Change test_error_after_conversions dependencies to local classes/variables