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/mbyte.c b/src/mbyte.c
index 5e6aed0..a9603cb 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -5634,8 +5634,7 @@
     if (l->lv_len == 0)
     {
 	// Clearing the table.
-	vim_free(cw_table);
-	cw_table = NULL;
+	VIM_CLEAR(cw_table);
 	cw_table_size = 0;
 	return;
     }