gh-140067: Fix memory leak in sub-interpreter creation (#140111) (#140261)

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. 

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_threading.py

@@ -1776,6 +1776,7 @@ def task():
         self.assertEqual(os.read(r_interp, 1), DONE)
 
     @cpython_only
+    @support.skip_if_sanitizer(thread=True, memory=True)
     def test_daemon_threads_fatal_error(self):
         import_module("_testcapi")
         subinterp_code = f"""if 1: