Fix bugs in zz_addi, zz_subi functions

[ssinai1]

These are corner case errors:
infinite recursion in zz_subi when c = LONG_MIN
and non-normalized result when a and c are both zero.
I fixed it by adding two new functions - not sure what the names should be (addu, addui...?).

[turtle-dragonfly]

Thank you for this fix. I would suggest adding some tests to exercise the issue(s).

I made one for myself, here: https://github.com/turtle-dragonfly/bsdnt/tree/test-inf-recursion-bug
Feel free to incorporate it (probably squash the commits).

[wbhart]

I agree. Thanks for the fix, but please consider adding some tests for the new functions. The general philosophy should be one of testing every function.

There should be tests for the original issue as well.