Return value of function which returns null is dereferenced without checking

Defect 103434 (STATIC_C)
Checker NULL_RETURNS (subcategory none)
File: /src/mongo/db/op_observer_impl.cpp
Function mongo::OpObserverImpl::onTransactionPrepare(mongo::OperationContext *)
/src/mongo/db/op_observer_impl.cpp, line: 873
Assigning: "session" = null return value from "get".

        Session* const session = OperationContextSession::get(opCtx);