PR IGNITE-1178 fix for NPE in GridCacheProcessor.onKernalStop()

[ALEKSEY KUZNETSOV]

Again, plz, review my PR : https://github.com/apache/ignite/pull/1517

https://issues.apache.org/jira/browse/IGNITE-1178
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*Best Regards,*

*Kuznetsov Aleksey*