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

(cherry picked from commit 992b031838)

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

libavcodec/mpegvideo.c

@@ -1658,7 +1658,7 @@ static void draw_line(uint8_t *buf, int sx, int sy, int ex, int ey,
         buf += sx + sy * stride;
         ex  -= sx;
         f    = ((ey - sy) << 16) / ex;
-        for (x = 0; x = ex; x++) {
+        for (x = 0; x <= ex; x++) {
             y  = (x * f) >> 16;
             fr = (x * f) & 0xFFFF;
             buf[y * stride + x]       += (color * (0x10000 - fr)) >> 16;