[3.14] gh-145376: Fix crashes in `md5module.c` and `hmacmodule.c` (GH-145422) (#145610)

gh-145376: Fix crashes in `md5module.c` and `hmacmodule.c` (GH-145422)

Fix a possible NULL pointer dereference in `md5module.c` and a double-free in `hmacmodule.c`.
Those crashes only occur in error paths taken when the interpreter fails to allocate memory.
(cherry picked from commit c1d77683213c400fca144692654845e6f5418981)

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>