[3.12] gh-127667: fix memory leaks in `hashlib` (GH-127668) (#130783)

gh-127667: fix memory leaks in `hashlib` (GH-127668)

- Correctly handle `NULL` values returned by `EVP_MD_CTX_md`.
- Correctly free resources in error branches.
- Consistently suppress `_setException()` return value when needed.
- Collapse `_setException() + return NULL` into a single statement.

(cherry-picked from commit 097846502b7f33cb327d512e2a396acf4f4de46e)