fbdev: atafb - add palette register check

Add check if palette register number is in correct range
for few drivers which miss it. The regno value comes
indirectly from user space.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
diff --git a/drivers/video/atafb.c b/drivers/video/atafb.c
index 37624f7..b7687c5 100644
--- a/drivers/video/atafb.c
+++ b/drivers/video/atafb.c
@@ -2242,6 +2242,9 @@
 	if (!external_vgaiobase)
 		return 1;
 
+	if (regno > 255)
+		return 1;
+
 	switch (external_card_type) {
 	case IS_VGA:
 		OUTB(0x3c8, regno);