Revert the assert in num_running_flushes to if

memtable/write_buffer_manager.cc

@@ -816,8 +816,9 @@ void WriteBufferManager::FlushEnded(bool /* wbm_initiated */) {
   // the WBM will not be aware of the number of running flushes at the time
   // it is enabled. The counter will become valid once all of the flushes
   // that were running when it was enabled will have completed.
-  assert(num_running_flushes_ > 0U);
-  --num_running_flushes_;
+  if (num_running_flushes_ > 0U) {
+    --num_running_flushes_;
+  }
 
   size_t curr_memory_used = memory_usage();
   RecalcFlushInitiationSize();