Fixed a bug in Compaction.cc where input_levels_ was not properly resized.

Summary:
Fixed a bug in Compaction.cc where input_levels_ was not properly resized.
Without this fix, there would be invalid access in input_levels_ when more
than two levels are involved in one compaction run.

This fix will go to master instead of compaction branch.

Test Plan: tested in compaction branch.

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20481