gh-154848: Enforce frame boundaries in the C unpickler (GH-154893)

gh-154848: Enforce frame boundaries in the C unpickler (GH-154893)

The C unpickler did not enforce PEP 3154 frame boundaries: an opcode or its
argument could straddle a frame, and a new frame could begin before the
previous one ended.  Such reads now raise UnpicklingError, as in the pure
Python implementation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>