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/time.c b/src/time.c
index bf6801f..272a5b5 100644
--- a/src/time.c
+++ b/src/time.c
@@ -313,7 +313,7 @@
     vimconv_T   conv;
     char_u	*enc;
 
-    vim_memset(&tmval, NUL, sizeof(tmval));
+    CLEAR_FIELD(tmval);
     fmt = tv_get_string(&argvars[0]);
     str = tv_get_string(&argvars[1]);