Potential NPE when closing a river

Seen in CI tests, it could happen that when closing a river, if indexer and slurper threads were not started, it causes a NPE when trying to interrupt them:

```
[2014-12-04 12:16:08,879][WARN ][river                    ] [node_s0] failed to delete river on stop [couchdb]/[elasticsearch_couch_test_test_scripting_quote_442659596819273583_1307630153]
java.lang.NullPointerException
at org.elasticsearch.river.couchdb.CouchdbRiver.close(CouchdbRiver.java:261)
at org.elasticsearch.river.RiversService.closeRiver(RiversService.java:202)
```

Closes #80.

(cherry picked from commit 851500b)