Fix deletion of uninitialized pointer

[jrbaxter0]

If a ChVisualSystemIrrlicht is constructed, then destructed without initializing, a segfault is caused due to deleteing the uninitialized m_receiver pointer. This fixes the problem.

It might be more elegant to instead initialize m_pointer with nullptr then check that here, but this seemed less invasive.

[rserban]

Thanks for the fix.! I actually would prefer initializing m_receiver to nullptr. Would you mind amending the PR?
Oh, and while you're at it, please move the definition of the destructor right after the constructor. :-)

[jrbaxter0]

Should be all set