Fix TimeoutMsgFromProto reported in issue 129

[meling]

When initializing the TimeoutMsg.MsgSignature field it required that the TimeoutMsg's ViewSignature was non-nil instead of checking for the MsgSignature being non-nil.

It is not clear to me if this can happen in practice, but this fixes the problem.

Thanks to @ikmOfSA for reporting the bug.

Fixes #129