Always check cookie before evaluation of READY flag to prevent misinterpretation of invalid DPM content

In seldom cases the current validation of the handshake cell content may not suffice (!= 0xFFFFFFFF && != 0x0BAD0BAD) and invalid data content is unintentionally interpreted as READY flag.
Thus, before evaluation of the READY flag the cookie at the very beginning of the DPM should be always checked, to make sure that we are dealing with valid DPM content.