stb_truetype: division by 0 in stbtt__fill_active_edges_new()

[ocornut]

Describe the bug
Apparently there's a division by zero in stbtt__fill_active_edges_new() on that line:

dy = (y_final - y_crossing ) / (x2 - (x1+1)); // if denom=0, y_final = y_crossing, so y_final <= y_bottom

See: ocornut/imgui#5139

To Reproduce
I am sorry I do not have a repro yet. This was reported by @mgerhardy and I am waiting on their answer. I tried loading a few fonts with an extra assert and it didn't trigger for me.

Martin proposed fix is vengi-voxel/vengi@a337eca but I haven't looked at the code so know if it makes sense.

[nothings]

I believe the suggested fix is fine, as the 0 case should mean there will be no further use of dy anyway.

[nothings]

I've fixed this locally, but I don't know when the next release will be.

[ocornut]

Thank you Sean.
(I've pushed the same fix on our own copy too now).