fixed dereferency sampler variable

Author: haoxian2

src/acl_kernel.cpp

@@ -642,6 +642,10 @@ CL_API_ENTRY cl_int CL_API_CALL clSetKernelArgIntelFPGA(cl_kernel kernel,
         kernel->arg_is_ptr[arg_index] = CL_FALSE;
       } else if (is_sampler) {
         cl_sampler sampler = *(cl_sampler *)arg_value;
+        if (sampler == 0) {
+          ERR_RET(CL_INVALID_ARG_VALUE, context, "Sampler value is NULL");
+        }
+
         int sampler_bitfield = 0;
 
         sampler_bitfield = 0;