datapath-windows: BSOD when disabling the extension

datapath-windows: BSOD when disabling the extension

When the filter detach routine is called while there are packets
still in processing, the OvsUninitSwitchContext function call will
decrement the switch context reference count without releasing the
switch context structure. This behaviour is correct and expected,
but the BSOD is caused in this case because the gOvsSwitchContext
variable is set to NULL, which is wrong.

The gOvsSwitchContext global variable must be set to NULL only when
the switch context structure is actually released.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/80
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>