lib: fix guard expression in timer.unref()

Fixes the following assertion on slow systems, like our ARM buildbot:

    $ out/Debug/node test/simple/test-timers-unref.js
    node: ../src/async-wrap-inl.h:101: v8::Handle<v8::Value>
    node::AsyncWrap::MakeCallback(uint32_t, int,
    v8::Handle<v8::Value>*): Assertion `cb_v->IsFunction()' failed.
    Aborted

The reason it only manifests on slow systems is that the test starts
a 1 ms interval timer, then defers timer.unref.bind({}) to the next
tick.  On fast systems, the test completes in under a millisecond,
before the callback is called.

This commit makes timer.unref() check that the receiver actually has
a timeout callback property.

Fixes #13.

R=@rvagg