Fix invalid instantiation and possibly unsafe accesses of array in class base_uint<BITS>

[pavlosantoniou]

The implementation of base_uint::operator++(int) and base_uint::operator--(int) is now safer.
Array pn is accessed via index i after bounds checking has been performed on the index, rather than before.

The logic of the while loops has also been made more clear.

[laanwj]

As I see it, this code can cause an out-of-bounds access only in the case of WIDTH=0, which would be pointless.
Or am I missing something?
I do agree that this change makes the code somewhat more readable.

[pavlosantoniou]

Yes, the out-of-bounds access occurs in the edge case of WIDTH == 0.
This template class can, although it probably won't, be instantiated with BITS < 32, something that would lead to WIDTH == 0.
I was mainly interested in ensuring the out-of-bounds-check before array access regardless of such assumptions.
Making it easier to follow the code logic is an extra gain.

[laanwj]

This template class can, although it probably won't, be instantiated with BITS < 32

Indeed - as the code throughout the entire class is implemented, it can only be instantiated for a positive multiple of 32. Might make sense to add a compile-time assertion for that.

[pavlosantoniou]

Do you think adding:
static_assert(BITS/32 > 0 && BITS%32 == 0, "Template parameter BITS must be a positive multiple of 32.");
in the class constructor would be fine?

[laanwj]

Yes, that'd be nice!

[pavlosantoniou]

I have added the assertions, is there something more I can do for this PR?

[pavlosantoniou]

Reopened with addition of static_assert for BITS value.

[sipa]

utACK