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/sign.c b/src/sign.c
index fb7d569..576c4d0 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -489,7 +489,7 @@
     sign_T		*sp;
     buf_T		*buf = wp->w_buffer;
 
-    vim_memset(sattr, 0, sizeof(sign_attrs_T));
+    CLEAR_POINTER(sattr);
 
     FOR_ALL_SIGNS_IN_BUF(buf, sign)
     {