ofproto: Don't destroy mutex before its use.

ofproto: Don't destroy mutex before its use.

Currently, we are calling guarded_list_destroy()
to destroy a mutex and then go ahead and use it through
delete_group
 ->delete_group__
   ->handle_flow_mod__
    ->run_rule_executes
     ->guarded_list_pop_all

The group related unit tests cause ovs-vswitchd to crash
because of this (on windows).

Calling guarded_list_destroy() after delete_group() solves the
problem.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>