otr: add missing return on error path

Due to a missing return, corrupt TLV data would cause an infinite loop
that consumes memory, eventually crashing the process.

Thanks to Ivan Markin for pointing this out.

Change-Id: Iaaf7c5f7ce911d3e542b86f6b942b15e2ccbdf3b
Reviewed-on: https://go-review.googlesource.com/35247
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

otr/otr.go

@@ -943,6 +943,7 @@ func (c *Conversation) processData(in []byte) (out []byte, tlvs []tlv, err error
 			t.data, tlvData, ok3 = getNBytes(tlvData, int(t.length))
 			if !ok1 || !ok2 || !ok3 {
 				err = errors.New("otr: corrupt tlv data")
+				return
 			}
 			tlvs = append(tlvs, t)
 		}