Simplify logic for freeing a NULL pointer. While no null-pointer
de-reference was performed, this wasn't as so obvious as the pointer
was passed to MEM_lockfree_allocN_len before checking for NULL.
NOTE: T99744 claimed the a NULL pointer free was a vulnerability,
while I can't see evidence for this - exiting early makes it clearer
the memory isn't accessed.
*Details*
- Add MEMHEAD_LEN macro, avoids redundant NULL check.
- Use "UNLIKELY(..)" hint's for error cases
(freeing NULL pointer and checking if `leak_detector_has_run`).
49babc7caa