Cleanup: early exit MEM_lockfree_freeN when called with NULL pointer

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`).

intern/guardedalloc/intern/mallocn_lockfree_impl.c

@@ -44,6 +44,7 @@ enum {
 #define PTR_FROM_MEMHEAD(memhead) (memhead + 1)
 #define MEMHEAD_ALIGNED_FROM_PTR(ptr) (((MemHeadAligned *)ptr) - 1)
 #define MEMHEAD_IS_ALIGNED(memhead) ((memhead)->len & (size_t)MEMHEAD_ALIGN_FLAG)
+#define MEMHEAD_LEN(memhead) ((memhead)->len & ~((size_t)(MEMHEAD_ALIGN_FLAG)))
 
 /* Uncomment this to have proper peak counter. */
 #define USE_ATOMIC_MAX
@@ -78,8 +79,8 @@ print_error(const char *str, ...)
 
 size_t MEM_lockfree_allocN_len(const void *vmemh)
 {
-  if (vmemh) {
-    return MEMHEAD_FROM_PTR(vmemh)->len & ~((size_t)(MEMHEAD_ALIGN_FLAG));
+  if (LIKELY(vmemh)) {
+    return MEMHEAD_LEN(MEMHEAD_FROM_PTR(vmemh));
   }
 
   return 0;
@@ -87,14 +88,11 @@ size_t MEM_lockfree_allocN_len(const void *vmemh)
 
 void MEM_lockfree_freeN(void *vmemh)
 {
-  if (leak_detector_has_run) {
+  if (UNLIKELY(leak_detector_has_run)) {
     print_error("%s\n", free_after_leak_detection_message);
   }
 
-  MemHead *memh = MEMHEAD_FROM_PTR(vmemh);
-  size_t len = MEM_lockfree_allocN_len(vmemh);
-
-  if (vmemh == NULL) {
+  if (UNLIKELY(vmemh == NULL)) {
     print_error("Attempt to free NULL pointer\n");
 #ifdef WITH_ASSERT_ABORT
     abort();
@@ -102,6 +100,9 @@ void MEM_lockfree_freeN(void *vmemh)
     return;
   }
 
+  MemHead *memh = MEMHEAD_FROM_PTR(vmemh);
+  size_t len = MEMHEAD_LEN(memh);
+
   atomic_sub_and_fetch_u(&totblock, 1);
   atomic_sub_and_fetch_z(&mem_in_use, len);