[api-minor] Let Catalog_getPageIndex check that the Ref actually points to a /Page dictionary

Currently the `getPageIndex` method will happily return `0`, even if the `Ref` parameter doesn't actually point to a proper /Page dictionary.
Having the API trust that the consumer is doing the right thing seems error-prone, hence this patch which adds a check for this case.

Given that the `Catalog_getPageIndex` method isn't used in any hot part of the codebase, this extra check shouldn't be a problem.
(Note: in the standard viewer, it is only ever used from `PDFLinkService_navigateTo` if a destination needs to be resolved during document loading, which isn't common enough to be an issue IMHO.)

test/unit/primitives_spec.js

@@ -1,5 +1,5 @@
 /* globals expect, it, describe, beforeEach, Name, Dict, Ref, RefSet, Cmd,
-           jasmine, isDict */
+           jasmine, isDict, isRefsEqual */
 
 'use strict';
 
@@ -158,4 +158,18 @@ describe('primitives', function() {
       expect(isDict(dict, 'Page')).toEqual(true);
     });
   });
+
+  describe('isRefsEqual', function () {
+    it('should handle different Refs pointing to the same object', function () {
+      var ref1 = new Ref(1, 0);
+      var ref2 = new Ref(1, 0);
+      expect(isRefsEqual(ref1, ref2)).toEqual(true);
+    });
+
+    it('should handle Refs pointing to different objects', function () {
+      var ref1 = new Ref(1, 0);
+      var ref2 = new Ref(2, 0);
+      expect(isRefsEqual(ref1, ref2)).toEqual(false);
+    });
+  });
 });