Note that the XRef cache will only hold objects returned through `Parser.getObj`, and indirectly via `Lexer.getObj`. Since neither of those methods will ever return `undefined`, we can simply `assert` that when inserting objects into the cache and thus get rid of one function call when doing cache lookups.
Obviously this won't have a huge effect on performance, however `XRef.fetch` is usually called *a lot* in larger documents and this patch thus cannot hurt.
c3b1c8f857