patch 9.0.1391: "clear" macros are not always used

Problem:    "clear" macros are not always used.
Solution:   Use ALLOC_ONE, VIM_CLEAR, CLEAR_POINTER and CLEAR_FIELD in more
            places. (Yegappan Lakshmanan, closes #12104)
diff --git a/src/list.c b/src/list.c
index 7118454..ae9e3e9 100644
--- a/src/list.c
+++ b/src/list.c
@@ -1687,7 +1687,7 @@
     list_T  *l = &sl->sl_list;
     int	    i;
 
-    memset(sl, 0, sizeof(staticList10_T));
+    CLEAR_POINTER(sl);
     l->lv_first = &sl->sl_items[0];
     l->lv_u.mat.lv_last = &sl->sl_items[9];
     l->lv_refcount = DO_NOT_FREE_CNT;