ptg tests: add null pointer

allow get null pointer via getPointer helper function
ptg tests: add more tests
ptg tests: add PTG.insertDerefBoth
ptg: fix applyRule(deref var asgn deref var)

I need to fix the graph from which I add the points-to pairs. (In the
case the topology of the graph is changed by the addition, I can not
use the new topology but I need to use the old one)