ext/gmp: gmp_invert addressing todo.

[devnexen]

No description provided.

[nielsdos]

This breaks the following test case (previously this threw an exception, now it returns false):

<?php
$zero = new GMP(0);
var_dump(gmp_invert(5, $zero));

And since CI didn't catch this, perhaps this should be a regression test?

[devnexen]

good idea will add this one.

[nielsdos]

It seems correct to me.