tree-wide: add missing assertions

Found by coccinelle.

src/sysinstall/sysinstall.c

@@ -369,8 +369,13 @@ static int acquire_device_list(
                 sd_varlink **link,
                 char ***ret_menu,
                 char ***ret_accepted) {
+
         int r;
 
+        assert(link);
+        assert(ret_menu);
+        assert(ret_accepted);
+
         r = connect_to_repart(link);
         if (r < 0)
                 return r;