Fix potential segmentation fault.

[nicolasnoble]

Described in #490.

The testcase described there now yields the following error instead of segfaulting:

Error: Cannot call getConnectivityState on a closed Channel
    at Immediate.checkState [as _onImmediate] (/home/pixel/sources/example_for_issue_grpc/node_modules/grpc/src/client.js:723:35)
    at runCallback (timers.js:810:20)
    at tryOnImmediate (timers.js:768:5)
    at processImmediate [as _immediateCallback] (timers.js:745:5)

We may also want to add testcases for this, but let's stop the bleeding first.

[nicolasnoble]

I'll be unavailable for a week, feel free to amend this PR if need be, it's editable by maintainers.