If a rct transaction would cause no change to be generated, a zero
change output is added, and sent to a randomly generated address.
This ensures that no transaction will be sent with just one output,
which could cause the receiver to be able to determine which of the
inputs in the sent rings is the real one.
This is very rare, since it requires the sum of outputs to be equal
to the sum of outputs plus the fee, which is now a function of the
last few blocks.
6c44f5c6eb