Bluetooth: SCO: check for codecs->num_codecs == 1 before assigning to sco_pi(sk)->codec

copy_struct_from_sockptr() fill 'buffer' in
sco_sock_setsockopt() with zeros, so there's no
real problem.

But it actually looks strange to do this,
without checking all of codecs->codecs[0]
really comes from userspace:

sco_pi(sk)->codec = codecs->codecs[0];

As only optlen < sizeof(struct bt_codecs) is checked
and codecs->num_codecs is not checked against != 1,
but only <= 1, and the space for the additional struct bt_codec
is not checked.

Note I don't understand bluetooth and I didn't do any runtime
tests with this! I just found it when debugging a problem
in copy_struct_from_sockptr().

I just added this to check the size is as expected:

BUILD_BUG_ON(struct_size(codecs, codecs, 0) != 1);
BUILD_BUG_ON(struct_size(codecs, codecs, 1) != 8);

And made sure it still compiles using this:

make CF=-D__CHECK_ENDIAN__ W=1ce C=1 net/bluetooth/sco.o

Fixes: 3e643e4efa1e ("Bluetooth: Improve setsockopt() handling of malformed user input")
Cc: Michal Luczaj <mhal@rbox.co>
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: David Wei <dw@davidwei.uk>
Cc: linux-bluetooth@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>