fix crypto/sha256.Sum224 panic

[boyvinall]

I was getting the same as #115:

panic: in crypto/sha256.Sum224: cannot convert slice t0[:] ([]byte) to *[28]byte

but found that updating golang.org/x/tools seemed to fix it. Still just tinkering around, I've not used this in anger yet, but looking forward to doing so as it looks really useful. Hope this helps someone.

[medyagh]

I was getting the same as #115:

panic: in crypto/sha256.Sum224: cannot convert slice t0[:] ([]byte) to *[28]byte

but found that updating golang.org/x/tools seemed to fix it. Still just tinkering around, I've not used this in anger yet, but looking forward to doing so as it looks really useful. Hope this helps someone.

this PR fixes this issue for me
#115

@ofabry any chance this could be merged into master?

[ondrajz]

Fixed in #121