Coverity analysis defect 49093: Unchecked dynamic_cast

A new defect has been detected and assigned to spencer in Coverity Connect.
http://coverity.mongodb.com//sourcebrowser.htm?projectId=10001#mergedDefectId=49093
The defect was flagged by checker FORWARD_NULL in
file /src/mongo/client/parallel.cpp
function mongo::ParallelSortClusteredCursor::setupVersionAndHandleSlaveOk(boost::shared_ptr<mongo::ParallelConnectionState>, const mongo::Shard &, boost::shared_ptr<mongo::Shard>, const mongo::NamespaceString &, const std::basic_string<char, std::char_traits<char>, std::allocator<char>>&, boost::shared_ptr<const mongo::ChunkManager>)
and this ticket was created by asya