Fix count of compacting update tasks started.

js/src/jsgc.cpp

@@ -2516,8 +2516,8 @@ GCRuntime::updateAllCellPointersParallel(MovingTracer* trc)
         for (i = 0; i < taskCount && !bgArenas.done(); ++i) {
             bgTasks[i].init(rt, &bgArenas, lock);
             startTask(bgTasks[i], gcstats::PHASE_COMPACT_UPDATE_CELLS);
+            tasksStarted++;
         }
-        tasksStarted = i;
 
         fgTask.init(rt, &fgArenas, lock);
     }