avoid seg fault on freeing cdo data with 0 size

examples/core_benchmark.c

@@ -378,10 +378,11 @@ int main(int argc, char *argv[]) {
       status = mstro_finalize();
 
       /* free allocated data*/
+      if (size_CDO != 0) {
 	      for(int i = 0; i < num_CDOs; i++){
 		      free(CDO_data[i]);
 	      }
-
+      }
       assert(status == MSTRO_OK);
     }