FloodSub is not started

[richardschneider]

When running the pubsub tests (ipfs/js-ipfs#108), I sometimes see the following error.

Debug: internal, implementation, error
    AssertionError [ERR_ASSERTION]: FloodSub is not started
    at FloodSub.unsubscribe (C:\Users\Owner\Documents\GitHub\js-ipfs\node_modules\libp2p-floodsub\src\index.js:324:5)
    at Object.unsubscribe (C:\Users\Owner\Documents\GitHub\js-ipfs\src\core\components\pubsub.js:41:12)
    at unsubscribe (C:\Users\Owner\Documents\GitHub\js-ipfs\src\http\api\resources\pubsub.js:36:19)
    at Object.internals.handler (C:\Users\Owner\Documents\GitHub\js-ipfs\node_modules\podium\lib\index.js:287:26)

[richardschneider]

The above comment is wrong. The stack trace indicates that unsubscribe is the problem.

This commit fixes it.