video: fbdev: metronomefb: fix some off by one bugs

The "mem" buffer has "size" bytes.  The ">" should be ">=" to prevent
reading one character beyond the end of the array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>