Fix SIGSEGV in travis

Summary:
Travis build was failing a lot. For example see https://travis-ci.org/facebook/rocksdb/builds/31425845

This fixes it.

Also, please don't put any code after SignalAll :)

Test Plan: no more SIGSEGV

Reviewers: yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D21417

db/db_impl.cc

@@ -2022,13 +2022,13 @@ void DBImpl::BackgroundCallFlush() {
     if (madeProgress || bg_schedule_needed_) {
       MaybeScheduleFlushOrCompaction();
     }
+    RecordFlushIOStats();
     bg_cv_.SignalAll();
     // IMPORTANT: there should be no code after calling SignalAll. This call may
     // signal the DB destructor that it's OK to proceed with destruction. In
     // that case, all DB variables will be dealloacated and referencing them
     // will cause trouble.
   }
-  RecordFlushIOStats();
 }
 
 void DBImpl::BackgroundCallCompaction() {