mpegvideo: fix loop condition in draw_line()

The loop condition `x = ex' is incorrect.  It should be `x <= ex'.

This bug was introduced in commit c65dfac4 "mpegvideo.c: K&R formatting
and cosmetics."

CC:libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>