Only skip parsing a stream in Parser_makeFilter when we know for sure that it is empty (PR 6372 follow-up)

For PDF files with multiple `/Filter`s, where the `/Length` entry is zero, we fail to render the file correctly. The reason is that `maybeLength` is `null` for the every filter except the first, and `!maybeLength` is thus truthy.
Hence it seems that we should completely ignore the `/Length` entry and also explicitly check `maybeLength === 0`.

Note that I've not (yet) come across a PDF file with this issue in the wild, but given all the stupid things PDF generators do I wouldn't be surprised if such a file actually exists. In order to prevent a possible future bug, I'm submitting this patch which includes a hand-edited PDF file that we currently cannot render correctly (but e.g. Adobe Reader can).

src/core/parser.js

@@ -564,7 +564,10 @@ var Parser = (function ParserClosure() {
       return stream;
     },
     makeFilter: function Parser_makeFilter(stream, name, maybeLength, params) {
-      if (stream.dict.get('Length') === 0 && !maybeLength) {
+      // Since the 'Length' entry in the stream dictionary can be completely
+      // wrong, e.g. zero for non-empty streams, only skip parsing the stream
+      // when we can be absolutely certain that it actually is empty.
+      if (maybeLength === 0) {
         warn('Empty "' + name + '" stream.');
         return new NullStream(stream);
       }

test/pdfs/.gitignore

@@ -65,6 +65,7 @@
 !simpletype3font.pdf
 !sizes.pdf
 !javauninstall-7r.pdf
+!multiple-filters-length-zero.pdf
 !issue3205r.pdf
 !issue3207r.pdf
 !issue3263r.pdf

test/test_manifest.json

@@ -1544,6 +1544,13 @@
        "lastPage": 1,
        "type": "eq"
     },
+    {  "id": "multiple-filters-length-zero",
+       "file": "pdfs/multiple-filters-length-zero.pdf",
+       "md5": "c273c3a6fb79cbf3034fe1b62b204728",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue5752",
        "file": "pdfs/issue5752.pdf",
        "md5": "aa20ad7cff71e9481c0cd623ddbff3b7",