First frame when entering a new level is wrong

[vanfanel]

Hi,

I have noticed that the first frame that appears after the wipe effect when loading a level is wrong: shows a distorted image, with some columns different than the others.

Looking at https://github.com/libretro/libretro-prboom/blob/master/src/f_wipe.c, I can't make head nor tail of how this can happen, could the wipe effect code causing this or is the problem elsewhere?