ipld::read does not use offset properly

[arajasek]

Given a block id, ipld::read reads a block from the kernel’s BlockRegistry into a provided buffer. ipld::read also accepts an offset into the block where the read should begin.

offset is passed into Kernel::block_read, which first calculates the length of the read to perform before copying block data to the output buffer:

let len = if offset as usize >= data.len() {
  0
} else {
  buf.len().min(data.len())
};

// ...

if len != 0 {
  buf.copy_from_slice(&data[offset as usize..][..len]);
}

Neither operation correctly handles nonzero offsets:

• The read length should be buf.len().min(data.len() - offset)
• The copy operation should be buf[..len].copy_from_slice(...)