Fix a SIGSEGV in BackgroundFlush

Summary:
This one wasn't easy to find :)

What happens is we go through all cfds on flush_queue_ and find no cfds to flush, *but* the cfd is set to the last CF we looped through and following code assumes we want it flushed.

BTW @sdong do you think we should also make BackgroundFlush() only check a single cfd for flushing instead of doing this `while (!flush_queue_.empty())`?

Test Plan: regression test no longer fails

Reviewers: sdong, rven, yhchiang

Reviewed By: yhchiang

Subscribers: sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30591

db/db_impl.cc

@@ -1801,17 +1801,18 @@ Status DBImpl::BackgroundFlush(bool* madeProgress, JobContext* job_context,
   ColumnFamilyData* cfd = nullptr;
   while (!flush_queue_.empty()) {
     // This cfd is already referenced
-    cfd = PopFirstFromFlushQueue();
+    auto first_cfd = PopFirstFromFlushQueue();
 
-    if (cfd->IsDropped() || !cfd->imm()->IsFlushPending()) {
+    if (first_cfd->IsDropped() || !first_cfd->imm()->IsFlushPending()) {
       // can't flush this CF, try next one
-      if (cfd->Unref()) {
-        delete cfd;
+      if (first_cfd->Unref()) {
+        delete first_cfd;
       }
       continue;
     }
 
     // found a flush!
+    cfd = first_cfd;
     break;
   }