datapath-windows: Avoid BSOD when switch context is NULL

I came around a BSOD that happened when trying to access pidHashLock
from the gOvsSwitchContext, which was NULL. The stop happened in
OvsAcquirePidHashLock function.

To reproduce this BSOD, make sure the extension is enabled and running,
disable it and, after that, execute 'ovs-dpctl.exe show'. The BSOD is
triggered afterwards.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/53
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>