core/cfg: cast pointer to long to silent compiler warning

- GH #3208

(cherry picked from commit 26785f8)
(cherry picked from commit 8aceadf)
(cherry picked from commit e90c246)

src/core/cfg/cfg_ctx.c

@@ -284,7 +284,7 @@ static int cfg_update_defaults(cfg_group_meta_t	*meta,
 				meta->array = array;
 				clone_done = 1;
 			}
-			if(ginst->vars + var->offset) {
+			if((unsigned long)ginst->vars + var->offset) {
 				memcpy(ginst->vars + var->offset, new_val, cfg_var_size(var));
 			} else {
 				LM_ERR("invalid variable offset\n");