[AST][ObjC] Fix crash when printing invalid objc categories

Summary:
If no valid interface definition was found previously we would crash.

With this change instead we just print `<<error-type>>` in place
of the NULL interface. In the future this could be improved by
saving the invalid interface's name and using that.

Reviewers: sammccall, gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83513