Fix panic on Windows

[btoews]

I'm hoping that github/certstore#10 will fix #33. This branch is building smimesign with that branch from the certstore repo.

@maufl would you mind downloading the version of smimesign building from this branch and checking if this fixes the error you were seeing? You can find the build artifacts here.

[maufl]

Looks good. Doesn't crash anymore and provides a good error message.

$ git commit -S
failed to get idenity signer: failed to load identity private key: failed to get private key for certificate
error: gpg failed to sign the data
fatal: failed to write commit object

[btoews]

Great. Thanks for your help in debugging this.

[maufl]

Thanks for fixing it so quickly!