CDRIVER-3535 introduced a leak in the topologyVersion check here:
https://github.com/mongodb/mongo-c-driver/commit/857d9de942b01ad48b1baa4d5ca2bb0cd05b4c96#diff-e697725d889d95aad42f045b28d66faeR1956

If the SDAM monitoring topology_changed callback is set, prev_td will have been allocated, and is not destroyed.