When looking briefly at using `Number.isInteger`/`Number.isNan` rather than `isInt`/`isNaN`, I noticed that there's a couple of not entirely straightforward cases to consider.
At first I really couldn't understand why `parseInt` is being used like it is in `XRef.fetchUncompressed`, since the `num` and `gen` properties of an object reference should *always* be integers.
However, doing a bit of code archaeology pointed to PR 4348, and it thus seem that this was a very deliberate change. Since I didn't want to inadvertently introduce any regressions, I've kept the `parseInt` calls intact but moved them to occur *only* when actually necessary.[1]
Secondly, I noticed that there's a redundant `isCmd` check for an edge-case of broken operators. Since we're throwing a `FormatError` if `obj3` isn't a command, we don't need to repeat that check.
In practice, this patch could perhaps be considered as a micro-optimization, but considering that `XRef.fetchUncompressed` can be called *many* thousand times when loading larger PDF documents these changes at least cannot hurt.
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[1] I even ran all tests locally, with an added `assert(Number.isInteger(obj1) && Number.isInteger(obj2));` check, and everything passed with flying colours.
However, since it appears that this was in fact necessary at one point, one possible explanation is that the failing test-case(s) have now been replaced by reduced ones.
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