video: Check for valid FB pointer before clearing

This command will start erasing at memory address zero
if there is not a valid framebuffer address that was found
during video_init().

This is a common case with Chrome OS devices in normal mode
when we do not execute the video option rom in coreboot.
Signed-off-by: NDuncan Laurie <dlaurie@chromium.org>
Signed-off-by: NSimon Glass <sjg@chromium.org>

drivers/video/cfb_console.c

@@ -2293,6 +2293,8 @@ int video_get_screen_columns(void)
 
 void video_clear(void)
 {
+	if (!video_fb_address)
+		return;
 #ifdef VIDEO_HW_RECTFILL
 	video_hw_rectfill(VIDEO_PIXEL_SIZE,	/* bytes per pixel */
 			  0,			/* dest pos x */