Fix null deref with y_offset in nk_group and nk_listview

[mtijanic]

We hit a rare null deref on y_offset in nk_group_scrolled_offset_begin(), that I think happens like this:

    // snippet from nk_group_begin_titled()

    x_offset = nk_find_value(win, id_hash);
    if (!x_offset) {
        x_offset = nk_add_value(ctx, win, id_hash, 0);
        y_offset = nk_add_value(ctx, win, id_hash+1, 0);
        NK_ASSERT(x_offset);
        NK_ASSERT(y_offset);
        if (!x_offset || !y_offset) return 0;
        *x_offset = *y_offset = 0;
    } else y_offset = nk_find_value(win, id_hash+1);
    return nk_group_scrolled_offset_begin(ctx, x_offset, y_offset, title, flags);

First, we don't find the x_offset, so we go into the if() branch. There, we manage to add x_offset but not y_offset. This causes it to bail early. Then, next frame, it will find x_offset and go into the else branch. There, it fails to find y_offset, and eventually calls into nk_group_scrolled_offset_begin() with y_offset = NULL.

Never got a local repro so can't say for sure if the existing NK_ASSERT(y_offset) was firing, but end user reports that this patch fixes it.

I think this is one of the issues that was reported in #513

[RobLoach]

Good find, thanks a lot. Two things...

1. Make sure to update the change in the src directory too. Otherwise the next time the paq.sh file is run, we'll lose this change.
2. Also, what do you think about updating this pattern in both nuklear_group.c and nuklear_list_view.c too? Could probably run into the same issue there, as it's using a similar pattern.

[mtijanic]

1. Ah, right, sorry! I just ported the change from the vendored file in my project, I didn't read up on how development actually goes here. Sorry. Do I need to also edit nuklear.h or will that be handled automatically when you pack it up?
2. Yep, seems similar. Do you want the same change there as well, or do you have some nicer way to generalize it in mind?

Thanks!

[RobLoach]

1. No need to appologize. Can be confusing. You don't absolutely need to edit nuklear.h. Anyone can run the script and it'll get updated eventually. Whichever is easier for you
2. Same way is fine 🤷

[mtijanic]

Done. I left it as smaller commits, but let me know if you prefer a squash. Or just squash when merging.