Fix SIGSEGV in compaction picker

Summary:
The SIGSEGV was introduced by https://reviews.facebook.net/D15171

I also fixed ExpandWhileOverlapping() which returned the failure by setting it's own stack variable to nullptr (!). This bug is present in 2.6 release, so I guess ExpandWhileOverlapping never fails :)

Test Plan: `make check`. Also MarkCallaghan confirmed it fixed the SIGSEGV he reported.

Reviewers: MarkCallaghan, kailiu, sdong, dhruba, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15261