Return value of function which returns null is dereferenced without checking

Defect 100050 (STATIC_C)
Checker NULL_RETURNS (subcategory none)
File: /src/mongo/db/commands/mr.cpp
Function mongo::mr::<unnamed>::collectionCount(mongo::OperationContext *, const std::_cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> &, bool)
/src/mongo/db/commands/mr.cpp, line: 617
Assigning: "db" = null return value from "get".

            Database* db = dbHolder().get(txn, ns);