Bug 1724107, be more precise when counting the number of allocator users, r=KrisWright

The patch is based on code inspection.

One could move sAllocatorUsers also to Free(), but shouldn't matter in practise.

Differential Revision: https://phabricator.services.mozilla.com/D121868

xpcom/threads/TimerThread.cpp

@@ -149,6 +149,7 @@ class nsTimerEvent final : public CancelableRunnable {
   }
   void operator delete(void* aPtr) {
     sAllocator->Free(aPtr);
+    sAllocatorUsers--;
     DeleteAllocatorIfNeeded();
   }
 
@@ -163,7 +164,6 @@ class nsTimerEvent final : public CancelableRunnable {
     MOZ_ASSERT(!sCanDeleteAllocator || sAllocatorUsers > 0,
                "This will result in us attempting to deallocate the "
                "nsTimerEvent allocator twice");
-    sAllocatorUsers--;
   }
 
   TimeStamp mInitTime;