mysqld: fix race in Work_queue

Summary:
In the dequeue loop, we check both if the queue is empty and the thread
isn't killed.

The latter is a problem. If the thread is in fact killed, then we
proceed to dequeue an item from an empty queue.

Notice that the queue had size of -1:
```
(lldb) p *((Sql_cmd_dump_table::Dump_worker_args*)0x00007f8386a86240)->queue
(Work_queue<Sql_cmd_dump_table::Dump_work_item>) {
  queue = size=4294967295 {}  // -1
  ...
  is_shutdown = true

(lldb) p thd->killed
(std::atomic<THD::killed_state>)  (_M_i = KILL_CONNECTION)
```

Differential Revision: D64731166

fbshipit-source-id: 25ef5371a7e49880a8dff18f69392fa8066b0735

include/work_queue.h

@@ -79,10 +79,10 @@ class Work_queue {
     PSI_stage_info old_stage;
     thd->ENTER_COND(&has_work, &lock, &stage_waiting_for_work_item, &old_stage);
 
-    if (queue.empty() && !thd->is_killed()) {
+    if (queue.empty()) {
       // Wait for work with timeout.
       do {
-        if (is_shutdown) {
+        if (is_shutdown || thd->is_killed()) {
           // Queue is empty and shutdown. No more work will be enqueued.
           goto exit;
         }
@@ -93,7 +93,7 @@ class Work_queue {
         thd_wait_begin(thd, THD_WAIT_SLEEP);
         mysql_cond_timedwait(&has_work, &lock, &abstime);
         thd_wait_end(thd);
-      } while (queue.empty() && !thd->is_killed());
+      } while (queue.empty());
     }
 
     assert(!queue.empty());