Check listener is still present when broadcasting
Check listener is still present when broadcasting

* Add check for `listener` in QueryManager's `broadcastQueries()`

It is possible the deletion of a query listener in `stopQuery()`
and the `forOwn` iteration of query listeners in `broadcastQueries()` to
overlap, causing a race condition. This commit explicitly checks that
the listener exists before attempting to invoke it.

apollostack/apollo-client,calebmer/apollo-client,stevewillard/apollo-client,apollographql/apollo-client,convoyinc/apollo-client,stevewillard/apollo-client,cesarsolorzano/apollo-client,calebmer/apollo-client,cesarsolorzano/apollo-client,stevewillard/apollo-client,calebmer/apollo-client,apollostack/apollo-client,convoyinc/apollo-client,apollostack/apollo-client,apollographql/apollo-client,convoyinc/apollo-client,cesarsolorzano/apollo-client