media: firewire: firedtv-avc: fix a buffer overflow in avc_ca_pmt()

The bounds checking in avc_ca_pmt() is not strict enough.  It should
be checking "read_pos + 4" because it's reading 5 bytes.  If the
"es_info_length" is non-zero then it reads a 6th byte so there needs to
be an additional check for that.

I also added checks for the "write_pos".  I don't think these are
required because "read_pos" and "write_pos" are tied together so
checking one ought to be enough.  But they make the code easier to
understand for me.  The check on write_pos is:

	if (write_pos + 4 >= sizeof(c->operand) - 4) {

The first "+ 4" is because we're writing 5 bytes and the last " - 4"
is to leave space for the CRC.

The other problem is that "length" can be invalid.  It comes from
"data_length" in fdtv_ca_pmt().

Cc: stable@vger.kernel.org
Reported-by: Luo Likang <luolikang@nsfocus.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
