crash in base class lookup due to infinitive recursion

Typing a somewhat ill-formed construct crashes the editor due to infinitive nested type lookup recursion. Typing the following in an empty source file manifests this:

class C : public C

The reason seems to be the initialization of variable 'binding' to 'this' at LookupContext.cpp:795, which in case no qualified base class name is found ends up calling the same object recursively infinitely. Instead initializing the variable to '0' seems to fix the problem.

After the change 'qtcreator -test CppTools' passes all tests.