datapath-windows: BSOD for a transactional NL cmd without dump state.

The BSOD is triggered by the incorect extraction of the OVS message
from the received input buffer from userspace, in _FlowNlGetCmdHandler
function.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/54
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>