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/os_win32.c b/src/os_win32.c
index a7a8ad1..1132bbe 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -950,7 +950,7 @@
     if (pker->UChar != 0)
 	return 1;
 
-    vim_memset(abKeystate, 0, sizeof (abKeystate));
+    CLEAR_FIELD(abKeystate);
 
     // Clear any pending dead keys
     ToUnicode(VK_SPACE, MapVirtualKey(VK_SPACE, 0), abKeystate, awAnsiCode, 2, 0);