Currently we rely on some undefined behavior when Object->cast_to() gets
called with a Null pointer. This used to work fine with GCC < 6 but
newer versions of GCC remove all codepaths in which the this pointer is
Null. However, the non-static cast_to() was supposed to be null safe.
This patch makes cast_to() Null safe and removes the now redundant Null
checks where they existed.
It is explained in this article: https://www.viva64.com/en/b/0226/
-Added new 3D stream player node
-Added ability for Area to capture sound from streams
-Added small features in physics to be able to properly guess distance to areas for sound
-Fixed 3D CollisionObject so shapes are added the same as in 2D, directly from children
-Fixed KinematicBody API to make it the same as 2D.
