if (!node.replSet || !!strcmp (node.replSet, replSet)) {
   MONGOC_INFO("%s: Got replicaSet \"%s\" expected \"%s\".",
               iter->host_and_port, node.replSet, replSet);
}

Error condition in the above listed code will cause a crash for node.replSet == null condition while trying tot print that value.

This is generally unsafe and should rather be something like {{ (node.replSet ? node.replSet : "<null>") }} in the MONGOC_INFO.