Fix Custom Menu save notices when there are no menus created

[nofearinc]

When adding the Custom Menu widget on a clean 3.9 install without custom menus created upfront, two PHP notices are triggered:

PHP Notice: Undefined index: title in /opt/lampp/htdocs/wp39/wp-includes/default-widgets.php on line 1326, referer: ​http://localhost/wp39/wp-admin/widgets.php
PHP Notice: Undefined index: nav_menu in /opt/lampp/htdocs/wp39/wp-includes/default-widgets.php on line 1327, referer: ​http://localhost/wp39/wp-admin/widgets.php

Adding an isset() check with default values before populating.

[nacin]

This doesn't seem to be a regression. Otherwise... related/duplicate of #27878? (At least the solution there.)

[nofearinc]

Replying to nacin:

This doesn't seem to be a regression. Otherwise... related/duplicate of #27878? (At least the solution there.)

UX-wise it seems like a good idea adding a "Select" which would solve part of the problem. However the notice was also triggered for the title which is a text input.

[ocean90]

Is this still an issue? Can't reproduce this in current trunk.

The other update() methods don't do an isset() check too.

[tyxla]

This issue has been addressed in 28433, related ticket: #22400.