Update ChunkedStream.makeSubStream to actually check if (some) data exists when the length parameter is undefined

Note how `XRef.fetchUncompressed`, which is used *a lot* for most PDF documents, is calling the `makeSubStream` method without providing a `length` argument.
In practice this results in the `makeSubStream` method, on the `ChunkedStream` instance, calling the `ensureRange` method with `NaN` as the end position,  thus resulting in no data being requested despite it possibly being necessary.

This may be quite bad, since in this particular case it will lead to a new `ChunkedStream` being created *and* also a new `Parser`/`Lexer` instance. Given that it's quite possible that even the very first `Parser.getObj` call could throw `MissingDataException`, this could thus lead to wasted time/resources (since re-parsing is necessary once the data finally arrives).

You obviously need to be very careful to not have `ChunkedStream.makeSubStream` accidentally requesting the *entire* file, hence its `this.end` property is of no use here, but it should be possible to at least check that the `start` of the data is present before any potentially expensive parsing occurs.

src/core/chunked_stream.js

@@ -234,7 +234,20 @@ class ChunkedStream {
   }
 
   makeSubStream(start, length, dict) {
-    this.ensureRange(start, start + length);
+    if (length) {
+      this.ensureRange(start, start + length);
+    } else {
+      // When the `length` is undefined you do *not*, under any circumstances,
+      // want to fallback on calling `this.ensureRange(start, this.end)` since
+      // that would force the *entire* PDF file to be loaded, thus completely
+      // breaking the whole purpose of using streaming and/or range requests.
+      //
+      // However, not doing any checking here could very easily lead to wasted
+      // time/resources during e.g. parsing, since `MissingDataException`s will
+      // require data to be re-parsed, which we attempt to minimize by at least
+      // checking that the *beginning* of the data is available here.
+      this.ensureByte(start);
+    }
 
     function ChunkedStreamSubstream() {}
     ChunkedStreamSubstream.prototype = Object.create(this);