Minor bug: no exception for a free unknown enum symbol

[dumblob]

The code

type E = enum<a, b, c>
x = E.b
io.writeln(x == $m ? 'equal' : 'not_equal')

prints not_equal instead of throwing an exception that $m doesn't exist. It might be related to https://github.com/daokoder/dao/issues/470 and/or https://github.com/daokoder/dao/issues/485 .