[3.14] gh-140067: Fix memory leak in sub-interpreter creation (GH-140111) (#140118)

[3.14] gh-140067: Fix memory leak in sub-interpreter creation  (GH-140111) (#140118)

* [3.14] gh-140067: Fix memory leak in sub-interpreter creation  (GH-140111)

Fix memory leak in sub-interpreter creation caused by overwriting of the previously used `_malloced` field. Now the pointer is stored in the first word of the memory block to avoid it being overwritten accidentally.
(cherry picked from commit 59547a251f7069dc6e08cb6082dd21872671e381)

Co-authored-by: Shamil <ashm.tech@proton.me>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>