Fix null dereference in yaml::Document::skip
Summary: The attached test case replicates a null dereference crash in
yaml::Document::skip(). This was fixed by adding a check and early
return in the method.
Reviewers: Bigcheese, hintonda, beanz
Reviewed By: hintonda
Subscribers: hiraditya, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D69974