chore: help Coverty avoid generating a false positive.

[rwgk]

Description

Please see #4814 for background.

This PR does not change the pybind11 behavior at all when NDEBUG is set.

For Coverty use -UNDEBUG.

The variable name change was helpful in determining that we think of the Coverty result as a false positive. The name change could be reverted, but is useful to keep (increases readability).

Suggested changelog entry:

An ``assert()`` was added to help Coverty avoid generating a false positive.

[henryiii]

Does current_overload cause some sort of wrapping? Why remove the last three chars?

[rwgk]

Does current_overload cause some sort of wrapping? Why remove the last three chars?

Changed to current_overload.

The longer name is great, I don't have a strong preference one way or another.

[rwgk]

Thanks Henry!