datapath-windows: Remove all duplicate checks for NULL.

Right now the gOvsSwitchContext pointer is checked against NULL
in a lot of places of the OVS extension code. This check should
be done only once to avoid wasteful checks. Thus I have added the
check in the dispatch routine, before doing any processing, and
removed all other checks from the rest of the code.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>