Function taxonomy_options_list() should check if function exists

We had this problem after disable Taxonomy Access Module in our Sites, every node edit thrown a "function not found".

The Problem is, that in taxonomy_options_list() the function saved in the $field['settings']['options_list_callback'] is not checked if it exists.

The attached patch adds an additional check with function_exists().

Of course the original problem comes from Taxonomy Access Module which doesn't removes this setting from the DB when disabled.
But probably Core should also have a check for this.