media: v4l2-tpg-core: avoid buffer overflows

Fix the following warnings:
	drivers/media/common/v4l2-tpg/v4l2-tpg-core.c:1146 gen_twopix() error: buffer overflow 'buf[1]' 8 <= 8
	drivers/media/common/v4l2-tpg/v4l2-tpg-core.c:1152 gen_twopix() error: buffer overflow 'buf[1]' 8 <= 8

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>