Fix unlink chunk with mismatched lengths status taking minutes

Hello!

Just found what I think is a bug. When you try to unlink a chunk that has the status mismatched lengths, and the length attribute of the chunks is ridiculous big, it can take minutes to finish the call because of how the requiereblocks() calculate the block size.

Here is small fix for that problem.

Please, if you don't like the fix, feel free to implement it in any way you like.