ofproto: Don't use connmgr after destruction.

ofproto: Don't use connmgr after destruction.

Set ofproto's connmgr pointer to NULL after the connmgr has been
destructed, and check for NULL when sending a flow removed
notification.

Verified by sending the flow removed message unconditionally and
observing numerous core dumps in the test suite.

Found by inspection.

Fixes: f695ebfae5 ("ofproto: Postpone sending flow removed messages.")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>