dgram: fix abort when getting `fd` of closed dgram

v8's `messages.js` file's `CallSiteGetMethodName` is running through all
object properties and getter to figure out method name of function that
appears in stack trace. This run-through will also read `fd` property of
`UDPWrap` instance's javascript object, making `UNWRAP()` fail.

As a simple alternative to the test case above, one could just keep
reference to the dgram handle and try accessing `handle.fd` after it has
been fully closed.

fix #6536

(@indutny Don't forget to merge this in master after landing in v0.10).