supportbundle: fix nil pointer error

when iproute2 is not installed in docker image, and run `antctl supportbundle` in
antrea-agent, it will panic, this is because, collectAgent will return a nil pointer
for systemv1beta1.SupportBundle, and in `func (r *supportBundleREST) Create`,
antrea handles the nil pointer directly which leads to the panic.

Signed-off-by: Bin Liu <biliu@vmware.com>

pkg/apiserver/registry/system/supportbundle/rest.go

@@ -158,8 +158,8 @@ func (r *supportBundleREST) Create(ctx context.Context, obj runtime.Object, _ re
 			default:
 				r.cache = b
 			}
+			r.clean(ctx, b.Filepath, bundleExpireDuration)
 		}()
-		r.clean(ctx, b.Filepath, bundleExpireDuration)
 	}()
 
 	return r.cache, nil