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>

ssh/session.go

@@ -601,5 +601,12 @@ func (w Waitmsg) Lang() string {
 }
 
 func (w Waitmsg) String() string {
-	return fmt.Sprintf("Process exited with: %v. Reason was: %v (%v)", w.status, w.msg, w.signal)
+	str := fmt.Sprintf("Process exited with status %v", w.status)
+	if w.signal != "" {
+		str += fmt.Sprintf(" from signal %v", w.signal)
+	}
+	if w.msg != "" {
+		str += fmt.Sprintf(". Reason was: %v", w.msg)
+	}
+	return str
 }