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/kword_test.c b/src/kword_test.c
index 92ea052..5d509fc 100644
--- a/src/kword_test.c
+++ b/src/kword_test.c
@@ -30,7 +30,7 @@
     buf_T buf;
     int c;
 
-    vim_memset(&buf, 0, sizeof(buf));
+    CLEAR_FIELD(buf);
     p_enc = (char_u *)"utf-8";
     p_isi = (char_u *)"";
     p_isp = (char_u *)"";