ssh: tolerate malformed SSH_MSG_IGNORE messages

[alexandrejbr]

Is your feature request related to a problem? Please describe.
Some SSH implementations, like SSHJ, can send an arguably malformed SSH_MSG_IGNORE message, i.e. sending just the type and not sending the actually data string.

openssh seems to accept this and as we talked before you seem to want to be inline with openssh. If that is the case being less strict with this messages could be an option.

Describe the solution you'd like
This is not so much a feature request but more sharing the information and letting you decide what do you think it's best for the erlang ssh application, if to keep being this strict in this case or being more relaxed as openssh is. I don't have a strong opinion on what is the best.

I have implemented a simple fix that I will be sharing in a PR so it's easier for you in case you want to accept this.