mdata_destroy() is supposed to be idempotent

src/mdata.c

@@ -277,7 +277,8 @@ mdata_destroy(struct mdata **md_p)
 {
 	struct mdata	*md = *md_p;
 
-	_mdata_clear(md);
+	if (md)
+		_mdata_clear(md);
 	xfree(md);
 	*md_p = NULL;
 }

tests/check_mdata.c

@@ -193,6 +193,7 @@ setup_checked(void)
 void
 teardown_checked(void)
 {
+	mdata_destroy(&md);
 	mdata_destroy(&md);
 	ck_assert_ptr_eq(md, NULL);