vsprintf: fix special yet useful case for vsprintf where n < 1.
vsprintf: fix special yet useful case for vsprintf where n < 1.

reported by jaldhar.


veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi