media: av7110: prevent underflow in write_ts_to_decoder()

[ Upstream commit eed9496a0501357aa326ddd6b71408189ed872eb ]

The buf[4] value comes from the user via ts_play().  It is a value in
the u8 range.  The final length we pass to av7110_ipack_instant_repack()
is "len - (buf[4] + 1) - 4" so add a check to ensure that the length is
not negative.  It's not clear that passing a negative len value does
anything bad necessarily, but it's not best practice.

With the new bounds checking the "if (!len)" condition is no longer
possible or required so remove that.

Fixes: fd46d16d ("V4L/DVB (11759): dvb-ttpci: Add TS replay capability")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>