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/jpg.js

@@ -973,6 +973,7 @@ var JpegImage = (function JpegImageClosure() {
         });
       }
       this.numComponents = this.components.length;
+      return undefined;
     },
 
     _getLinearizedBlockData(width, height, isSourcePDF = false) {