BUG/MINOR: payload: reject a \0 byte in req.rdp_cookie

BUG/MINOR: payload: reject a \0 byte in req.rdp_cookie

A string sample cannot contain a \0 byte, but req.rdp_cookie returned
the cookie value as found in the RDP packet, up to the CR, so a client
could put a \0 in it. Such a value was then matched by "-m str" on its
prefix: "public\0x" matched "public".

Stop parsing the cookie value on a \0 byte and fail the fetch, as for a
malformed cookie. The same function is used by the rdp-cookie
persistence, so such a cookie is not used as a key anymore either.

A reg-test is added to check that a "mstshash=public\0x" cookie does
not match "public".

This must be backported to all stable branches.