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/clipboard.c b/src/clipboard.c
index e9e426f..7a35112 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -2033,7 +2033,7 @@
 	clear_oparg(&oa);
 	oa.regname = (cbd == &clip_plus ? '+' : '*');
 	oa.op_type = OP_YANK;
-	vim_memset(&ca, 0, sizeof(ca));
+	CLEAR_FIELD(ca);
 	ca.oap = &oa;
 	ca.cmdchar = 'y';
 	ca.count1 = 1;