gh-115891: Fix debug byte filling in free-threaded build

[colesbury]

The previous code had two bugs. First, the debug offset in the mimalloc heap includes the two pymalloc debug words, but the pointer passed to fill_mem_debug does not include them. Second, the current object heap is correct source for allocations, but not deallocations.

• Issue: test_capi failure when GIL disabled #115891

[colesbury]

!buildbot nogil

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @colesbury for commit 137e000 🤖

The command will test the builders whose names match following regular expression: nogil

The builders matched are:

• x86-64 MacOS Intel ASAN NoGIL PR
• x86-64 MacOS Intel NoGIL PR
• ARM64 MacOS M1 Refleaks NoGIL PR
• ARM64 MacOS M1 NoGIL PR
• AMD64 Ubuntu NoGIL Refleaks PR
• AMD64 Ubuntu NoGIL PR
• AMD64 Windows Server 2022 NoGIL PR

[DinoV]

LGTM!

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x has failed when building commit 75c6c05.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/345/builds/7275) and take a look at the build logs.
4. Check if the failure is related to this commit (75c6c05) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/345/builds/7275

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 4, done.        
remote: Counting objects:  25% (1/4)        
remote: Counting objects:  50% (2/4)        
remote: Counting objects:  75% (3/4)        
remote: Counting objects: 100% (4/4)        
remote: Counting objects: 100% (4/4), done.        
remote: Compressing objects:  33% (1/3)        
remote: Compressing objects:  66% (2/3)        
remote: Compressing objects: 100% (3/3)        
remote: Compressing objects: 100% (3/3), done.        
remote: Total 4 (delta 1), reused 4 (delta 1), pack-reused 0        
From https://github.com/python/cpython
 * branch                  main       -> FETCH_HEAD
Note: switching to '75c6c05fea212330f4b0259602ffae1b2cb91be3'.

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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 75c6c05fea gh-115891: Fix debug byte filling in free-threaded build (#116018)
Switched to and reset branch 'main'

configure: WARNING: pkg-config is missing. Some dependencies may not be detected correctly.

make: *** [Makefile:2147: buildbottest] Error 3