Add missing assert that task isn't queued

Probably won't fix anything but this assert is in queue()
but not queueAtFront() and inserting a task twice is Really Bad.

Bug: 36139852
Test:  ¯\_(ツ)_/¯
Change-Id: Ida0f829eecfdd46c17c36b816528c49d12b7cf29

libs/hwui/renderthread/RenderThread.cpp

@@ -98,6 +98,7 @@ void TaskQueue::queue(RenderTask* task) {
 }
 
 void TaskQueue::queueAtFront(RenderTask* task) {
+    LOG_ALWAYS_FATAL_IF(task->mNext || mHead == task, "Task is already in the queue!");
     if (mTail) {
         task->mNext = mHead;
         mHead = task;