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

util/auto_roll_logger.cc

@@ -47,7 +47,11 @@ void AutoRollLogger::Logv(const char* format, va_list ap) {
     if ((kLogFileTimeToRoll > 0 && LogExpired()) ||
         (kMaxLogFileSize > 0 && logger_->GetLogFileSize() >= kMaxLogFileSize)) {
       RollLogFile();
-      ResetLogger();
+      Status s = ResetLogger();
+      if (!s.ok()) {
+        // can't really log the error if creating a new LOG file failed
+        return;
+      }
     }
 
     // pin down the current logger_ instance before releasing the mutex.