USB: serial: digi_acceleport: fix incomplete rx sanity check

USB: serial: digi_acceleport: fix incomplete rx sanity check

commit 1b0aed2b1600f6e5c7b9acfbd610a4e351ef5232 upstream.

Make sure the received data has the required headers before parsing it.

Also drop the redundant urb-status check, which has already been handled
by the caller.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>