Fix GH-8778: Integer arithmethic with large number variants fails

When casting a `variant` to `int`, we need to heed the proper `zval`
type, which is an signed 64bit integer on x64, whil

Fix GH-8778: Integer arithmethic with large number variants fails

When casting a `variant` to `int`, we need to heed the proper `zval`
type, which is an signed 64bit integer on x64, while `VT_INT` is only
a signed 32bit integer.

Closes GH-8779.

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