Perform sitename title check

There is a bug in views_rss_core_preprocess_channel_title that causes the feed title field value to be displayed when the "Use the site name for the title" checkbox has been selected. Here are the steps to reproduce:

1) User deselects "Use the site name for the title" in the View title field and enters a feed title. View is saved.
2) User edits the View title, and selects the "Use the site name for the title" checkbox without clearing the feed title field. View is saved.
3) Even though the site name has been chosen for the title, the code never checks if the checkbox is selected. It simply checks if the feed title field has a value.

The attached patch fixes this error. Please review, thanks.