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.

Fixes #184.