Fixes BF-28895.

The GLIBCXX_DEBUG macros complain about making a copy of an invalidated iterator, whether you use it or not.

We copy a potentially invalid iterator here, but we don't use it unless other conditions indicate that the bound copy is still valid.
https://github.com/10gen/mongo/blob/0e6a660c41d4a07228837835f572daedd3683fc0/src/mongo/client/streamable_replica_set_monitor.cpp#L422

We don't use it, but the instrumentation doesn't know that. It just complains about the copy, so we can conditionally copy to fix this problem.