add comment to indicate that setting the pointer to null has no effect in sfree.

You need Memory::destroy() but that can't be applied to all use cases.

src/REAXFF/reaxff_tool_box.cpp

@@ -101,6 +101,6 @@ void sfree(void *ptr)
   if (ptr == nullptr) return;
 
   free(ptr);
-  ptr = nullptr;
+  ptr = nullptr;  // NOTE: this has no effect
 }
 }    // namespace ReaxFF