fix: read the correct number of bytes for the piece

[rvagg]

No description provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #17   +/-   ##
=========================================
  Coverage          ?   78.01%           
=========================================
  Files             ?        4           
  Lines             ?      191           
  Branches          ?        0           
=========================================
  Hits              ?      149           
  Misses            ?       27           
  Partials          ?       15           

Files	Coverage Δ
commp.go	57.89% <100.00%> (ø)
writer/writer.go	90.32% <ø> (ø)

[github-actions]

Suggested version: 2.0.1

Comparing to: v2.0.0 (diff)

Changes in configuration file(s):

(empty)

gorelease says:

# summary
Suggested version: v2.0.1

gocompat says:

HEAD is now at 1b7ad32 chore: update deps, add dependabot
Switched to branch 'master'
Your branch is up to date with 'origin/master'.

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[ribasushi]

Yes this looks identical to the original behavior of https://github.com/filecoin-project/go-commp-utils/blob/v0.1.4/ffiwrapper/ffiwrapper.go#L40-L54

Unsure if worth retracting 2.0.0 over this - fine either way.

Apologies I missed this eaerlier