!4180 memcheck包执行ci用例，json.cpp中json_build_array函数中存在溢出风险

Merge pull request !4180 from 姜岩/master

src/common/backend/utils/adt/json.cpp

@@ -1892,7 +1892,6 @@ Datum json_build_array(PG_FUNCTION_ARGS)
     appendStringInfoChar(result, '[');
     for (i = 0; i < nargs; i++) {
         val_type = get_fn_expr_argtype(fcinfo->flinfo, i);
-        arg = PG_GETARG_DATUM(i + 1);
         /* see comments in json_build_object above */
         if (val_type == UNKNOWNOID && get_fn_expr_arg_stable(fcinfo->flinfo, i)) {
             val_type = TEXTOID;