Questionable coding in nth_value

Currently Window function nth_value is coded as following:

    nth = DatumGetInt32(WinGetFuncArgCurrent(winobj, 1, &isnull));
    if (isnull)
        PG_RETURN_NULL();
    const_offset = get_fn_expr_arg_stable(fcinfo->flinfo, 1);

    if (nth <= 0)
        ereport(ERROR,
        :
        :

Is there any reason why argument 'nth' is not checked earlier?
IMO, it is more natural "if (nth <= 0)..." is placed right after "nth = DatumGetInt32...".

Attached is the patch which does this.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp
diff --git a/src/backend/utils/adt/windowfuncs.c b/src/backend/utils/adt/windowfuncs.c
index b87a624fb2..f4ff060930 100644
--- a/src/backend/utils/adt/windowfuncs.c
+++ b/src/backend/utils/adt/windowfuncs.c
@@ -696,15 +696,16 @@ window_nth_value(PG_FUNCTION_ARGS)
     int32        nth;
 
     nth = DatumGetInt32(WinGetFuncArgCurrent(winobj, 1, &isnull));
-    if (isnull)
-        PG_RETURN_NULL();
-    const_offset = get_fn_expr_arg_stable(fcinfo->flinfo, 1);
-
     if (nth <= 0)
         ereport(ERROR,
                 (errcode(ERRCODE_INVALID_ARGUMENT_FOR_NTH_VALUE),
                  errmsg("argument of nth_value must be greater than zero")));
 
+    if (isnull)
+        PG_RETURN_NULL();
+
+    const_offset = get_fn_expr_arg_stable(fcinfo->flinfo, 1);
+
     result = WinGetFuncArgInFrame(winobj, 0,
                                   nth - 1, WINDOW_SEEK_HEAD, const_offset,
                                   &isnull, NULL);