[3.13] gh-121460: Skip freeing unallocated arenas (gh-121589)

`munmap(NULL)` is not noop, like `free(NULL)` is.

Fixes an observed testsuite hang on 32-bit ARM systems.

(cherry picked from commit a802277914405786f6425f2776605c44bd407fc0, AKA gh-121491)

Co-authored-by: Stefano Rivera <stefano@rivera.za.net>