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/MouseJoint.cpp

@@ -31,7 +31,7 @@ namespace physics
 namespace box2d
 {
 	MouseJoint::MouseJoint(Body * body1, float x, float y)
-		: Joint(body1)
+		: Joint(body1), joint(NULL)
 	{
 		b2MouseJointDef def;