Fix OOB read in getlist() and putdata() when __len__ overstates length

[lazerg]

Fixes #9892.

Changes proposed in this pull request:

• getlist() and _putdata() read the sequence length before calling PySequence_Fast(). A custom sequence whose __len__ reports more items than __getitem__ actually produces makes the later loop read past the materialized list, which segfaults.
• Both functions now take the length from the materialized PySequence_Fast result instead, so they never index past what was actually built.