patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)
diff --git a/src/menu.c b/src/menu.c
index 3728aa7..310cf59 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -727,8 +727,7 @@
 	menup = &menu->children;
 	parent = menu;
 	name = next_name;
-	vim_free(dname);
-	dname = NULL;
+	VIM_CLEAR(dname);
 	if (pri_tab[pri_idx + 1] != -1)
 	    ++pri_idx;
     }