x/crypto/ssh: omit empty fields in error message

Although the signal and msg fields are assigned together, their
values originate from the remote server and may be empty.

Fixes golang/go#14251

Change-Id: I9d9094cc69f3c14bf1648af59951f6b6c7a71e0a
Reviewed-on: https://go-review.googlesource.com/22196
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>