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:
* not at all sure 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
This is a false positive. f() is static.