Don't check the this pointer for 0 in member functions

Don't check the this pointer for 0 in member functions

This actually violates the C++ standard that defines that
you aren't allowed to call member functions on an invalid
object.

Instead insert the 0 pointer checks on the caller side where
required.

Change-Id: I8be3c3831594bb6482e9ef6de6e590ec437ac0f8
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>