Fix bug where we would not check if there was actually a page to navigate to when using arrow keys

Author: azerupi

src/theme/book.js

@@ -32,11 +32,15 @@ $( document ).ready(function() {
         switch (e.keyCode) {
             case KEY_CODES.NEXT_KEY:
                 e.preventDefault();
-                window.location.href = $('.nav-chapters.next').attr('href');
+                if($('.nav-chapters.next').length) {
+                    window.location.href = $('.nav-chapters.next').attr('href');
+                }
                 break;
             case KEY_CODES.PREVIOUS_KEY:
                 e.preventDefault();
-                window.location.href = $('.nav-chapters.previous').attr('href');
+                if($('.nav-chapters.previous').length) {
+                    window.location.href = $('.nav-chapters.previous').attr('href');
+                }
                 break;
         }
     });