Add stricter double free checking on large allocs (#550)

Although, double frees are generally caught on large allocs.  This is
done very late in the process after many operations more operations have
occurred.

This change brings that check much earlier in the process.

Author: mjp41

src/snmalloc/mem/localalloc.h

@@ -629,6 +629,7 @@ namespace snmalloc
 
       const PagemapEntry& entry =
         Config::Backend::get_metaentry(address_cast(p_tame));
+
       if (SNMALLOC_LIKELY(local_cache.remote_allocator == entry.get_remote()))
       {
 #  if defined(__CHERI_PURE_CAPABILITY__) && defined(SNMALLOC_CHECK_CLIENT)
@@ -647,6 +648,11 @@ namespace snmalloc
 #  if defined(__CHERI_PURE_CAPABILITY__) && defined(SNMALLOC_CHECK_CLIENT)
         dealloc_cheri_checks(p_tame.unsafe_ptr());
 #  endif
+
+        // Detect double free of large allocations here.
+        snmalloc_check_client(
+          !entry.is_backend_owned(), "Memory corruption detected");
+
         // Check if we have space for the remote deallocation
         if (local_cache.remote_dealloc_cache.reserve_space(entry))
         {