Return value of function which returns null is dereferenced without checking

Defect 72719 (STATIC_C)
Checker NULL_RETURNS (subcategory none)
File: /src/mongo/client/replica_set_monitor_read_preference_test.cpp
Function <unnamed>::selectNode(const std::vector<mongo::ReplicaSetMonitor::SetState::Node, std::allocator<mongo::ReplicaSetMonitor::SetState::Node>> &, mongo::ReadPreference, const mongo::TagSet &, int, bool *)
/src/mongo/client/replica_set_monitor_read_preference_test.cpp, line: 67
Dereferencing a null pointer "set.findNode(out)".

                *isPrimarySelected = !out.empty() && set.findNode(out)->isMaster;