Fix blinking square when attacking on a pet-vehicle for sabre mount mana-collection world quest

[spiralofhope]

• On a sabre mount, when doing the world quest to collect mana -- reproduced for Suramar.
• Select an enemy unit
• Run past it and press 1 (attack)
• Run into a wall

There is a box, presumably the vehicle/pet attack button, which is empty and blinks to the right of the main area.

Fstack revealed no information about this. It was as though the square didn't exist. This isn't supposed to be possible! Perhaps try with /fstack true

This oddity may be revealed in other ways, but this was the only way I could reproduce it.

[spiralofhope]

This would have to be re-tested for 8.x.

[spiralofhope]

I don't particularly care to test this obscure problem.