Summary: It looks like ASAN with gcc 4.9 works better than 4.8.1. It detected this possibility of heap buffer overflow. This was in our codebase for a year :)
Test Plan: COMPILE_WITH_ASAN=1 make backupable_db && ./backupable_db
Reviewers: yhchiang, rven, sdong
Reviewed By: sdong
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D32085