An Adult Red Dragon is within melee range of a fighter with the Sentinel feat.
On its turn, the dragon moves away, and the fighter takes an attack of opportunity, reducing the dragon's speed to 0.
From the Sentinel feat (PHB, page 169):
When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.
The fighter then takes his turn. At the end of the fighter's turn, the dragon takes the Wing Attack legendary action.
Wing Attack (Costs 2 Actions)
The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Am I right in assuming that the dragon wouldn't be able to move because its speed would be 0?