bpo-40955: Fix memory leak in subprocess module (GH-20825)

bpo-40955: Fix memory leak in subprocess module (GH-20825)

```
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    GH-0 0x7f008bf19667 in __interceptor_malloc (/lib64/libasan.so.6+0xb0667)
    GH-1 0x7f007a0bee4a in subprocess_fork_exec /home/heimes/dev/python/cpython/Modules/_posixsubprocess.c:774
    GH-2 0xe0305b in cfunction_call Objects/methodobject.c:546
```

Signed-off-by: Christian Heimes <christian@python.org>
(cherry picked from commit 0d3350daa8123a3e16d4a534b6e873eb12c10d7c)

Co-authored-by: Christian Heimes <christian@python.org>