Replace MACE_CHECK with MACE_CHECK_NOTNULL.

mace/kernels/activation.h

@@ -116,7 +116,7 @@ class ActivationFunctor {
     const T *input_ptr = input->data<T>();
     T *output_ptr = output->mutable_data<T>();
     if (activation_ == PRELU) {
-      MACE_CHECK(alpha != nullptr) << "PReLU's alpha parameter shouldn't be null";
+      MACE_CHECK_NOTNULL(alpha);
       const T *alpha_ptr = alpha->data<T>();
       PReLUActivation(input_ptr, output->size(), input->dim(3), alpha_ptr, output_ptr); 
     } else {

mace/kernels/opencl/activation_opencl.cc

@@ -62,7 +62,7 @@ void ActivationFunctor<DeviceType::OPENCL, T>::operator()(const Tensor *input,
     int idx = 0;
     kernel_.setArg(idx++, *(input->opencl_image()));
     if (activation_ == PRELU) {
-      MACE_CHECK(alpha != nullptr) << "PReLU's alpha parameter shouldn't be null";
+      MACE_CHECK_NOTNULL(alpha);
       kernel_.setArg(idx++, *(alpha->opencl_image()));
     }
     kernel_.setArg(idx++, static_cast<float>(relux_max_limit_));