Add NULL initializers for pointers.

Constructing invalid shapes, e.g. a rectangle with width 0, could result
in a b2Assert to fail. In turn, Shape::~Shape tried to destroy a the shape
with an uninitialized pointer, yielding a segfault. Add NULL-initializers
and checks for NULL to prevent this.

src/modules/physics/box2d/DistanceJoint.cpp

@@ -31,7 +31,7 @@ namespace physics
 namespace box2d
 {
 	DistanceJoint::DistanceJoint(Body * body1, Body * body2, float x1, float y1, float x2, float y2)
-		: Joint(body1, body2)
+		: Joint(body1, body2), joint(NULL)
 	{
 		b2DistanceJointDef def;
 		def.Initialize(body1->body, body2->body, world->scaleDown(b2Vec2(x1,y1)), world->scaleDown(b2Vec2(x2,y2)));