video: fbdev: fix OOB read in vga_8planes_imageblit()

commit bd018a6a75cebb511bb55a0e7690024be975fe93 upstream.

syzbot is reporting OOB read at vga_8planes_imageblit() [1], for
"cdat[y] >> 4" can become a negative value due to "const char *cdat".

[1] https://syzkaller.appspot.com/bug?id=0d7a0da1557dcd1989e00cb3692b26d4173b4132



Reported-by: syzbot <syzbot+69fbd3e01470f169c8c4@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/90b55ec3-d5b0-3307-9f7c-7ff5c5fd6ad3@i-love.sakura.ne.jp


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>