Fix Autoroll logger

Summary:
If auto roll logger can't create a new LOG file on roll (if, for example, somebody deletes rocksdb directory while rocksdb is running, khm), we'll try to call Logv on invalid address and get a SIGSEGV. This diff will fix the issue

Here's the paste of the stack trace: https://phabricator.fb.com/P8276386 (fb-only)

Test Plan: make check is fine, although not really testing error condition

Reviewers: haobo, ljin, yhchiang

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17367