Box2DJoint debug drawing issues

[briansemrau]

Error spam problem:
In some of the Box2DJoint debug draw functions, there are missing checks for if the joint is valid before trying to get certain values.

Cleanliness:
Debug drawing for joints contains lots of similar code for each joint. It would be nice to clean this up.

Transform problems:
At least when working with prismatic joints, the geometry drawn for the constraints between two rotating bodies is rotated at double the angle it should (simply: it draws in the wrong spot). This should be fixed alongside the cleanliness issue to prevent silly bugs like this when all the other joints get implemented.