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>