bpo-43362: Fix invalid free and return check in _sha3 module (GH-25463)

Commit 93d50a6a8d0c5d332c11aef267e66573a09765ac / GH-21855 changed the
order of variable definitions, which introduced a potential invalid free
bug. Py_buffer object is now initialized earlier and the result of
Keccak initialize is verified.

Co-authored-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Christian Heimes <christian@python.org>

Co-authored-by: Alex Henrie <alexhenrie24@gmail.com>