Every _free() function should be a macro that sets the corresponding pointer to NULL.

Our tor_free() macros has saved us a bunch of trouble in the past by ensuring that once we've freed a pointer, that pointer gets set to NULL.

Wouldn't it be cool if all of our free() calls did this? It should make our code generally less error-prone.

As we work on legacy/trac#23847 (moved), I bet we will find it extremely helpful.