test_confparse: verify that clearing a routerset sets it to NULL.

src/test/test_confparse.c

@@ -592,6 +592,10 @@ test_confparse_reset(void *arg)
   config_reset_line(&test_fmt, tst, "interval", 1);
   tt_int_op(tst->interval, OP_EQ, 10);
 
+  tt_ptr_op(tst->routerset, OP_NE, NULL);
+  config_reset_line(&test_fmt, tst, "routerset", 0);
+  tt_ptr_op(tst->routerset, OP_EQ, NULL);
+
  done:
   config_free(&test_fmt, tst);
 }