patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/memfile.c b/src/memfile.c
index 0d167e6..4fe3294 100644
--- a/src/memfile.c
+++ b/src/memfile.c
@@ -1337,7 +1337,7 @@
     static void
 mf_hash_init(mf_hashtab_T *mht)
 {
-    vim_memset(mht, 0, sizeof(mf_hashtab_T));
+    CLEAR_POINTER(mht);
     mht->mht_buckets = mht->mht_small_buckets;
     mht->mht_mask = MHT_INIT_SIZE - 1;
 }
@@ -1480,7 +1480,7 @@
 	 * a power of two.
 	 */
 
-	vim_memset(tails, 0, sizeof(tails));
+	CLEAR_FIELD(tails);
 
 	for (mhi = mht->mht_buckets[i]; mhi != NULL; mhi = mhi->mhi_next)
 	{