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/regexp.c b/src/regexp.c
index b4fb595..3911c90 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2014,7 +2014,7 @@
 		argv[0].v_type = VAR_LIST;
 		argv[0].vval.v_list = &matchList.sl_list;
 		matchList.sl_list.lv_len = 0;
-		vim_memset(&funcexe, 0, sizeof(funcexe));
+		CLEAR_FIELD(funcexe);
 		funcexe.argv_func = fill_submatch_list;
 		funcexe.evaluate = TRUE;
 		if (expr->v_type == VAR_FUNC)