Correct the addblock utility import.

This commit corrects the check in the addblock utility to check each block
links the previous hash as intended versus the current block.

Fixes #162.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.