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>