Fix crash in pthread_tsd_cleanup on macOS ARM64 (#1177)

Complete the NULL handling fix from commit 515047b by also checking for
NULL page entries in the 2-level page map lookup.

The issue occurs on macOS ARM64 during pthread TSD cleanup when
thread_local C++ objects are destroyed. During this late cleanup phase,
the TLS for mimalloc may already be invalidated, causing page map
lookups to return NULL for valid pointers.

Commit 515047b changed the sub==NULL case to return _mi_page_empty
instead of NULL, but missed the case where sub[sub_idx] is NULL.

Author: austenstone

include/mimalloc/internal.h

@@ -578,7 +578,9 @@ static inline mi_page_t* _mi_checked_ptr_page(const void* p) {
   const size_t idx = _mi_page_map_index(p, &sub_idx);
   mi_submap_t const sub = _mi_page_map[idx];
   if mi_unlikely(sub == NULL) return (mi_page_t*)&_mi_page_empty;
-  return sub[sub_idx];
+  mi_page_t* const page = sub[sub_idx];
+  if mi_unlikely(page == NULL) return (mi_page_t*)&_mi_page_empty;
+  return page;
 }
 
 #endif