Solomachine PubKey fixes

[AdityaSripal]

• Change GetPubKey() to not panic if the cached value is nil
• Determine what happens if we send multisignature to a single pubkey solomachine. Document or check pubkey is single pubkey

[cwgoes]

cc @colin-axner do you have any opinions here?

[colin-axner]

1. sounds reasonable to me.
2. An error is returned which is correct.

multiPK, ok := pubKey.(multisig.PubKey)
        if !ok {
            return sdkerrors.Wrapf(ErrSignatureVerificationFailed, "invalid pubkey type: expected %T, got %T", (multisig.PubKey)(nil), pubKey)
        }

Multsigs cause the public key to be casted to a multsig pk, this will fail and the error above will be returned.

[AdityaSripal]

2. You are talking about if we send a multisignature to a single pubkey. The issue is if we send a single signature to a multi pubkey (the inverse situation). In that case, VerifySignature will panic.

I suggest that we switch on pubkey type instead.

switch on multipubkey and then check inside that case that the signature is a multisignature before calling VerifyMultiSignature

Since I don't believe there is a separate interface for single pubkeys this can be the default case,
But inside the default case we should check signature is single-signature before calling VerifySignature.

[colin-axner]

Determine what happens if we send multisignature to a single pubkey solomachine

Sorry, I was going off of this.

Proposed solution sounds fine.