Fixing DescendantFonts object fetching (#600)

pdf.js

@@ -5190,7 +5190,7 @@ var PartialEvaluator = (function partialEvaluator() {
         if (isRef(df))
           df = xref.fetch(df);
 
-        dict = xref.fetch(isRef(df) ? df : df[0]);
+        dict = xref.fetchIfRef(isRef(df) ? df : df[0]);
 
         type = dict.get('Subtype');
         assertWellFormed(isName(type), 'invalid font Subtype');

test/test_manifest.json

@@ -43,6 +43,11 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "plusminus",
+       "file": "pdfs/Test-plusminus.pdf",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "openoffice-pdf",
        "file": "pdfs/DiwanProfile.pdf",
        "link": true,