Fix deadlock in background translation thread shutdown (#7239)

TryDequeue checks for _disposed before taking the lock.  If another
thread calls Dispose before it takes the lock, it won't get woken up by
the PulseAll call, and will deadlock in Monitor.Wait.

Double-checking _disposed with the lock taken should avoid this.

src/ARMeilleure/Translation/TranslatorQueue.cs

@@ -80,9 +80,12 @@ namespace ARMeilleure.Translation
                         return true;
                     }
 
+                    if (!_disposed)
+                    {
                         Monitor.Wait(Sync);
                     }
                 }
+            }
 
             result = default;