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/misc1.c b/src/misc1.c
index b4b74f7..92aa82f 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -631,7 +631,7 @@
 {
     char_u	buf[4];
 
-    vim_memset(buf, 0, sizeof(buf));
+    CLEAR_FIELD(buf);
 
     if (time_for_testing == 93784)
     {