At least one method on a native object is added as a getter,
namely `MessagePort.prototype.onmessage`. When a MessagePort
attempts to call this method from C++ in response to receiving
data, it will first invoke that getter and then call the function.

Since `worker.terminate()` interrupts execution, this means
that the getter may fail (without being faulty code on its own).
This means that at least one test exercising these methods in
combination has been flaky and could have crashed, because
we did not actually check that the getter returns a value
so far, resulting in dereferencing an empty `Local`.

The proper fix for this is to use the non-deprecated overload
of `Get()` and check the result like we should be doing.
Also, as a (related) fix, don’t crash if the method
is not a function but rather something else, like a getter
could provide.

Example test failure: https://ci.nodejs.org/job/node-test-commit-linux-containered/4976/nodes=ubuntu1604_sharedlibs_zlib_x64/console

    17:56:56 not ok 1955 parallel/test-worker-dns-terminate
    17:56:56   ---
    17:56:56   duration_ms: 1.237
    17:56:56   severity: crashed
    17:56:56   exitcode: -11
    17:56:56   stack: |-