add early exit to .pop()

- seems odd to perform work on a node that could potentially be a nullptr from the moment the function is called (regardless of CAS)
- same thing applies to `void pop(T &val)` (early exit allows the code the run without throwing for bad access to `old_head->next`)

Author: ITHelpDec

listings/listing_7.3.cpp

@@ -23,9 +23,9 @@ class lock_free_stack
     }
     std::shared_ptr<T> pop()
     {
+        if (!head_) { return std::shared_ptr<T>(); }
         node* old_head=head.load();
-        while(old_head &&
-              !head.compare_exchange_weak(old_head,old_head->next));
-        return old_head ? old_head->data : std::shared_ptr<T>();
+        while(!head.compare_exchange_weak(old_head,old_head->next));
+        return old_head->data;
     }
 };