Also check maybeLength when deciding if a stream is empty in Parser_makeFilter (issue 6360)

The problem with the PDF files in the issue, besides the obviously broken XRef tables which we're able to recover from, is that many/most of the streams have Dictionaries where the `Length` entry is set to `0`. This causes us to return `NullStream`, instead of the appropriate one in `Parser_makeFilter`.

Fixes 6360.

src/core/parser.js

@@ -524,7 +524,7 @@ var Parser = (function ParserClosure() {
       return stream;
     },
     makeFilter: function Parser_makeFilter(stream, name, maybeLength, params) {
-      if (stream.dict.get('Length') === 0) {
+      if (stream.dict.get('Length') === 0 && !maybeLength) {
         return new NullStream(stream);
       }
       try {

test/pdfs/issue6360.pdf.link

@@ -0,0 +1 @@
+http://web.archive.org/web/20150820205437/https://s3-eu-west-1.amazonaws.com/document-api-images-prod/docs/KNJfiZ5NV_Nb8d9VJmbvnWQ-5tlAir_arkwCEN_4EnA/application-pdf?AWSAccessKeyId=ASIAJUC4JIACNAMOVPXQ&Expires=1440104129&Signature=ItgaKidUAi70YSE%2F4E1Fbp8XSP0%3D&x-amz-security-token=AQoDYXdzEJH%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEa4AMvYGaEfeu06eu1RFUPRoTnDKITutvSW8T1REdKtxHHq41ZRI%2FQL5hbzkEsy7gamqnktaoSpft5PBdrv5btUERD8LOA2tnbrOCCbodE3JubVkzMp4FIuZ%2F%2Fkpb%2F%2Be3dfG36VjpMvLWTKkitAsrFCEy5MFTp9RY3aAF0fNT%2Buhfz797O4WKH4qRm6juVRnMhX1gBjiPg0ipftdg5ZyKcHa1hBq4C%2FNoDeU2aX5XM3HboVBlsaY5NkfJyk%2Bjj6tfJVMY8scNyzQumediNb6CyWSlAEg3QJ2MzdWplBvsye%2BRMn7uC0d1Zw8uc7jYPl9jtY4Ix2%2Fnzv9rrxUSOTzXWqnxTHc%2BfTrrgMhWmSUxff80vhaIk4RAHfPu3F38BomBWcE%2B71vn7I5NwLEwg6DQeytuWuz0fVpQ4W1qudqurZ%2F5rPhyyFK0UoXDSAc%2B14TMBpgBbsj0qSZLQl%2FSMPAcKEKwug9CAhlyyech5l8VraSIdrw3N0u%2F5TMsX5VJNFH3xXKcXdljMNWRmR8eTSpExdJRgjODlKR2VEn56gaLDTPFvpR2f4N7%2FCcun%2FbeYgt2g6wTOXTmEkRr8loMEJxQBXS86ulvIvAbjsc8dPbWKcRjfH4dLTtKA7SWRzP2hs9SJc5EgxfzXrgU%3D

test/test_manifest.json

@@ -1062,6 +1062,15 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "issue6360",
+       "file": "pdfs/issue6360.pdf",
+       "md5": "58c5455ffd84b1c07ad2d0fa90cd5e26",
+       "rounds": 1,
+       "link": true,
+       "firstPage": 1,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "issue2931",
        "file": "pdfs/issue2931.pdf",
        "md5": "ea40940eaf3541b312bda9329167da11",