panels_views_load_all() breaks when there is no panel_view

Due to several typos panels_views_load_all() breaks when there is no panel_view. Because of some typos introduced with http://drupal.org/cvs?commit=118536, in this case a NULL value is returned instead of an empty array. This leads on various places to warnings.
This also prohibited the newly introduced caching of panels_views_load_all() in a static variable.
Reason: The code inconsistently uses either $panel_views or $panels_views.

I decided to go for $panel_views and made sure this variable is consistently used in panels_views_load_all() and panels_views_load().