Enable the consistent-return ESLint rule

This rule is already enabled in mozilla-central, and helps ensure more consistent functions/methods, see https://searchfox.org/mozilla-central/rev/b9da45f63cb567244933c77b2c7e827a057d3f9b/tools/lint/eslint/eslint-plugin-mozilla/lib/configs/recommended.js#119-120

Please see https://eslint.org/docs/rules/consistent-return for additional information.

src/core/obj.js

@@ -266,6 +266,7 @@ class Catalog {
     } else if (this.catDict.has('Dests')) { // Simple destination dictionary.
       return this.catDict.get('Dests');
     }
+    return undefined;
   }
 
   get pageLabels() {
@@ -1596,7 +1597,7 @@ var XRef = (function XRefClosure() {
       }
 
       if (recoveryMode) {
-        return;
+        return undefined;
       }
       throw new XRefParseException();
     },