Validate ImageFileReader frame indices

[yinkev]

Summary

• reject negative frame indices and indices equal to NumberOfFrames
• validate before indexing the offset table or seeking into pixel data
• document the ValueError contract on raw and decoded frame access

Root cause

The previous condition used index > number_of_frames. It therefore allowed Python negative indexing and missed the first invalid upper-bound value, index == number_of_frames.

Validation

• regression fails on upstream/master
• focused regression: 1 passed
• full tests/test_io.py: 111 passed, 9 skipped
• Flake8 on touched files
• git diff --check

[yinkev]

Replaced by #444 to conform to this repository's documented contributor workflow and branch-naming convention. The replacement uses the same signed commit and identical code diff: #444