vgacon: Prevent faulty bootparams.screeninfo from causing harm

If a zero for the number of colums or rows manages to slip through,
gotoxy() will underflow vc->vc_pos, causing the next action on the
referenced memory to end with a page fault.

Make the check in vgacon_startup() more pessimistic to prevent that.
Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

drivers/video/console/vgacon.c

@@ -398,9 +398,8 @@ static const char *vgacon_startup(void)
 #endif
 	}
 
-	/* boot_params.screen_info initialized? */
-	if ((screen_info.orig_video_mode  == 0) &&
-	    (screen_info.orig_video_lines == 0) &&
+	/* boot_params.screen_info reasonably initialized? */
+	if ((screen_info.orig_video_lines == 0) ||
 	    (screen_info.orig_video_cols  == 0))
 		goto no_vga;