gh-135379: fix MSVC warning: conversion from 'stwodigits' to 'digit'

[chris-eibl]

Fixes MSVC warning C4244: 'initializing': conversion from 'stwodigits' to 'digit', possible loss of data.

• Issue: Top-of-stack caching in the JIT #135379

[Fidget-Spinner]

Just checking: what does the rest of longobject do? Sorry I cant check myself as Im on mobile

[chris-eibl]

There are many (digit) casts if something is assigned to a digit, but is a bigger data type.

Here that cast is missing, hence, MSVC fires the warning. Which is of no concern, since a few lines above

if(!is_medium_int(x)){returnPyStackRef_NULL}

ensures that the contents of stwodigits x is small enough to be cast into a digit without loss.

But since here a signed conversion takes place, too, I'd appreciate a second pair of eyes @markshannon ?