fix: Handle block offset exceeding chunk length in memchunk.go

[cyriltovena]

The code change in memchunk.go fixes a bug where the block offset plus the length exceeds the chunk length. This bug is addressed by adding a check to ensure that the offset plus length does not exceed the chunk length before accessing the byte slice.

This seems to happen only when a chunk is corrupted and the cause is still unknown.

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

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• Documentation added
• Tests updated
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