Once Demonspine Whip’s ability resolves and gives the +X/+0 bonus to a creature, that bonus stays with that creature for the rest of the turn even if Demonspine Whip moves to another creature or leaves the battlefield.

If Demonspine Whip is no longer on the battlefield by the time its first ability resolves, the game checks whether it was attached to a creature at the time it left the battlefield. If it was, that creature gets the +X/+0 bonus. If it wasn’t, no creature gets the bonus.

When the first ability resolves, the +X/+0 bonus is given to the creature that Demonspine Whip is attached to at that time, regardless of what creature (if any) it was attached to when the ability was activated. If Demonspine Whip is on the battlefield but not attached to a creature at this time, no creature gets the bonus.