go.crypto/ssh/terminal: support Go 1.0.

For those still stuck on Go 1.0.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11297043
diff --git a/terminal.go b/terminal.go
index f83be8c..411b1f1 100644
--- a/terminal.go
+++ b/terminal.go
@@ -568,6 +568,8 @@
 
 		t.remainder = t.inBuf[:n+len(t.remainder)]
 	}
+
+	panic("unreachable") // for Go 1.0.
 }
 
 // SetPrompt sets the prompt to be used when reading subsequent lines.