[3.10] gh-99612: Fix PyUnicode_DecodeUTF8Stateful() for ASCII-only data (GH-99613) (GH-107224) (#107230)

Previously *consumed was not set in this case.

(cherry picked from commit b8b3e6afc0a48c3cbb7c36d2f73e332edcd6058c)
(cherry picked from commit f08e52ccb027f6f703302b8c1a82db9fd3934270)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>