Handle multiple calls to DBImpl::PauseBackgroundWork() and DBImpl::ContinueBackgroundWork()

Summary: Handle multiple calls to DBImpl::PauseBackgroundWork() and DBImpl::ContinueBackgroundWork()

Test Plan: rocksdb.information_schema handles this case.

Reviewers: igor

Reviewed By: igor

Subscribers: hermanlee4, jkedgar, dhruba

Differential Revision: https://reviews.facebook.net/D50781

db/db_impl.cc

@@ -1850,6 +1850,9 @@ Status DBImpl::PauseBackgroundWork() {
 
 Status DBImpl::ContinueBackgroundWork() {
   InstrumentedMutexLock guard_lock(&mutex_);
+  if (bg_work_paused_ == 0) {
+    return Status::InvalidArgument();
+  }
   assert(bg_work_paused_ > 0);
   bg_work_paused_--;
   if (bg_work_paused_ == 0) {