frontend/validate: remove redundant variable assign

Stack here is used only to provide user with error message. So it
makes sence to move stack.ptr assign directly before error will be
throwed.

benchmark.lua shows a little speedup after such change.

Author: olegrok

avro_schema/frontend.lua

@@ -850,12 +850,11 @@ copy_data = function(stack, schema, data, visited)
             local fieldmap = get_record_field_map(schema)
             -- check if the data contains unknown fields
             for k, _ in pairs(data) do
-                stack.ptr[frame_no] = k
                 local field = schema.fields[fieldmap[k]]
                 if not field or field.name ~= k then
+                    stack.ptr[frame_no] = k
                     error('@Unknown field', 0)
                 end
-                stack.ptr[frame_no] = nil
             end
             -- copy data
             for _, field in ipairs(schema.fields) do