Fix verifying the sig when a derivation path is used

[notmandatory]

          Fix verifying the sig when a derivation path is used, currently only works if no path given.

Originally posted by @notmandatory in #19 (comment)

[notmandatory]

Waiting on related issue coinkite/coinkite-tap-proto#56.

[r1b2ns]

The PR #67 attempts to fix this