[3.13] gh-145376: Fix crashes in md5module.c (GH-145422) (#145611)

* gh-145376: Fix crashes in md5module.c

Fix a possible NULL pointer dereference in `md5module.c`.
This can only occur in error paths taken when the interpreter fails to allocate memory.

(cherry-picked from c1d77683213c400fca144692654845e6f5418981)

* 📜🤖 Added by blurb_it.

* Update Modules/md5module.c

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>