amifb: test virtual screen range before subtraction on unsigned

dx and dy are u32's, so the test should occur before the subtraction
Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Cc: Antonino Daplas <adaplas@pol.net>
Cc: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/amifb.c

@@ -2048,13 +2048,16 @@ static void amifb_copyarea(struct fb_info *info,
 	width = x2 - dx;
 	height = y2 - dy;
 
+	if (area->sx + dx < area->dx || area->sy + dy < area->dy)
+		return;
+
 	/* update sx,sy */
 	sx = area->sx + (dx - area->dx);
 	sy = area->sy + (dy - area->dy);
 
 	/* the source must be completely inside the virtual screen */
-	if (sx < 0 || sy < 0 || (sx + width) > info->var.xres_virtual ||
-	    (sy + height) > info->var.yres_virtual)
+	if (sx + width > info->var.xres_virtual ||
+			sy + height > info->var.yres_virtual)
 		return;
 
 	if (dy > sy || (dy == sy && dx > sx)) {