Detect -> accesses on uninitialized pointers

This detects `->` accesses via uninitialized pointers as discussed here https://github.com/danmar/cppcheck/pull/421#issuecomment-54943202

Some concerns:
* totally unsure if the loop which traverses the AST until it finds a suitable parent is fast enough in generic cases
* one of the testcases now emits two warnings, both correct, which means that the test has to expect them is some specific order, I'm not sure if that's good