otr: add missing return on error path
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 <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,ScriptRock/crypto,golang/crypto,ScriptRock/crypto,golang/crypto