Improvments for hook_entity_view implementation

Under certain circumstances, the current implementation of hook_entity_view throws some PHP warnings:

Warning: Invalid argument supplied for foreach() in dfp_entity_view() (line 174 of /modules/dfp/dfp.module).

Furthermore I think it's not really efficient to do that term category calculation on every entity view, even if this feature isn't activated at all (at least if I understood the configuration options correctly).

Attached patch fixes the PHP warning and ads a check if the term category feature is activated.