corrected preproc bug

src/c_interface/tensor_field.c

@@ -82,7 +82,7 @@ void tensor_field_deallocate(tensor_field field) {
 }
 
 void tensor_field_from_futhark_f32_4d_inplace(futhark_4d *f, struct futhark_context *ctx, tensor_field field) {
-#ifdef DEBUG
+#ifndef NDEBUG
     const int64_t *shape = futhark_shape_4d(ctx, f);
     fint nx = shape[0];
     fint ny = shape[1];