salsa20: panic when len(out) < len(in) following cipher.Stream

Change-Id: I5c8d603cc3ec844b8d44e51b3138bcf16e95c7fc
Reviewed-on: https://go-review.googlesource.com/112235
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>

salsa20/salsa20.go

@@ -32,7 +32,7 @@ import (
 // be either 8 or 24 bytes long.
 func XORKeyStream(out, in []byte, nonce []byte, key *[32]byte) {
 	if len(out) < len(in) {
-		in = in[:len(out)]
+		panic("salsa20: output smaller than input")
 	}
 
 	var subNonce [16]byte