xrow: reset parameters of decode_subscribe() to default

Nullify all the output parameters passed to xrow_decode_subscribe().

This should help us get rid of cases when some of the parameters were
left uninitialized when the SUBSCRIBE request didn't contain the
corresponding fields.

Also it's now possible to remove all the explicit parameter
initializations for xrow_decode_* functions using
xrow_decode_subscribe() internally. Do that as well.

Follow-up #6034