Fix for own recent reference count error.

- The armature weakref list was  being incref'd twice then decrefed twice (incref and decref were used incorrectly), now only once. My 'fix' broke this.
- In bpy_pydriver_create_dict the 2 refs added from running PyDict_SetItemString twice were undone when clearing the dictionary (added comment)
- changed Py_XDECREF to Py_DECREF int BPY_pyconstraint_update and BPY_pyconstraint_target, Py_XDECREF checs for NULL value which would have crashed blender before it got to Py_XDECREF anyway.
- after every error is reported (PyErr_Print), remove sys.last_traceback and clear the error, I found this fixed certain crashes (usually when starting the game engine or exiting blender), so best do this all the time.

- header_text.c, CcdPhysicsEnvironment.cpp, KX_CameraActuator.cpp - remove some warnings.

source/gameengine/Physics/Bullet/CcdPhysicsEnvironment.cpp

@@ -1346,7 +1346,7 @@ struct OcclusionBuffer
 	static int	clip(const btVector4* pi,btVector4* po)
 	{
 		btScalar	s[2*NP];
-		btVector4	pn[2*NP], *p;
+		btVector4	pn[2*NP];
 		int			i, j, m, n, ni;
 		// deal with near clipping
 		for(i=0, m=0;i<NP;++i)