otr: smpFailureError processing tlvTypeSMP3 prevents sending tlvTypeSMP4

* The other side is left hanging, waiting for their chance to do the
   comparison.

Change-Id: I186933ebb53a30e190414c5572206851681e0a37
Reviewed-on: https://go-review.googlesource.com/15260
Reviewed-by: Adam Langley <agl@golang.org>

otr/otr.go

@@ -417,12 +417,11 @@ func (c *Conversation) Receive(in []byte) (out []byte, encrypted bool, change Se
 					change = SMPSecretNeeded
 					c.smp.saved = &inTLV
 					return
-				} else if err == smpFailureError {
+				}
+				if err == smpFailureError {
 					err = nil
 					change = SMPFailed
-					return
-				}
-				if complete {
+				} else if complete {
 					change = SMPComplete
 				}
 				if reply.typ != 0 {