fix: reject truncated encoded integers

[wesen]

Summary

Return io.ErrUnexpectedEOF when memUvarintReader reaches the end of its
input after reading a continuation byte.

The reader previously checked for empty input only before entering its decode
loop. If the final available byte had its continuation bit set, the next loop
iteration indexed beyond the slice and panicked.

Change

• check the cursor before each in-loop slice access;
• preserve the consumed cursor position on truncation;
• add regression cases containing one and multiple continuation bytes.

Validation

go test ./...

Additional resources

If you are willing to endure some slop writing, this is how I/the llm encountered the issue: https://parc.yolo.scapegoat.dev/note/projects/2026/07/28/project-report-zapx-defensive-varint-decoding-for-corrupt-bleve-postings