Fix double free in merge-left rebalance.

This commit fixes a bug where deletions that caused merge-left rebalances were updating
the parent node which caused a node to "reappear" even after it had been deleted. This was
fixed in merge-right rebalances a while ago but merge-left is less frequent so it was
missed until now.

Many thanks to Jordan Sherer (@jsherer) for finding and reporting the bug.
Merge pull request #185 from benbjohnson/fix-bulk-delete

Fix double free in merge-left rebalance.