Add a bit more validation to `Catalog_readPageLabels`, to ensure that the Page Labels are well formed

src/core/obj.js

@@ -283,7 +283,6 @@ var Catalog = (function CatalogClosure() {
       var pageLabels = new Array(this.numPages);
       var style = null;
       var prefix = '';
-      var start = 1;
 
       var numberTree = new NumberTree(obj, this.xref);
       var nums = numberTree.getAll();
@@ -300,15 +299,16 @@ var Catalog = (function CatalogClosure() {
 
           var s = labelDict.get('S');
           assert(!s || isName(s), 'Invalid style in PageLabel dictionary.');
-          style = (s ? s.name : null);
+          style = s ? s.name : null;
 
-          var p = labelDict.get('P') || '';
-          assert(isString(p), 'Invalid prefix in PageLabel dictionary.');
-          prefix = stringToPDFString(p);
+          var p = labelDict.get('P');
+          assert(!p || isString(p), 'Invalid prefix in PageLabel dictionary.');
+          prefix = p ? stringToPDFString(p) : '';
 
-          start = labelDict.get('St') || 1;
-          assert(isInt(start), 'Invalid start in PageLabel dictionary.');
-          currentIndex = start;
+          var st = labelDict.get('St');
+          assert(!st || (isInt(st) && st >= 1),
+                 'Invalid start in PageLabel dictionary.');
+          currentIndex = st || 1;
         }
 
         switch (style) {