Destroy mutex on subchannel destruction

src/core/ext/filters/client_channel/subchannel.c

@@ -188,6 +188,7 @@ static void subchannel_destroy(grpc_exec_ctx *exec_ctx, void *arg,
   grpc_connector_unref(exec_ctx, c->connector);
   grpc_pollset_set_destroy(exec_ctx, c->pollset_set);
   grpc_subchannel_key_destroy(exec_ctx, c->key);
+  gpr_mu_destroy(&c->mu);
   gpr_free(c);
 }