Bump leveldb subtree

Summary:
Pull in changes from https://github.com/bitcoin/bitcoin/pull/11674#issuecomment-348174674.

Backport of Bitcoin Core PR12451
https://github.com/bitcoin/bitcoin/pull/12451

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4004