Summary:
ForwardIterator::SVCleanup() sometimes didn't pin superversion when it was supposed to. See the added test for the scenario. Here's the ASAN output of the added test without the fix (using `COMPILE_WITH_ASAN=1 make`): https://pastebin.com/9rD0Ywws
Closes https://github.com/facebook/rocksdb/pull/3415

Differential Revision: D6817414

Pulled By: al13n321

fbshipit-source-id: bc80c44ea78a3a1fa885dfa448a26111f91afb24