If the target creature is an illegal target by the time Garruk's second ability tries to resolve, the ability won't resolve. You won't draw a card. If the target is legal but not destroyed (most likely because it has indestructible), you will still draw.

If lethal damage is dealt to one of Garruk's Wolf tokens at the same time that Garruk's loyalty is brought to 0 or less, Garruk is put into your graveyard before the Wolf's triggered ability can save him.