media: v4l2-tpg: array index could become negative

commit e5f71a27fa12c1a1b02ad478a568e76260f1815e upstream.

text[s] is a signed char, so using that as index into the font8x16 array
can result in negative indices. Cast it to u8 to be safe.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by:  <syzbot+ccf0a61ed12f2a7313ee@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>      # for v4.7 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>