LP#1466990: Detailed search results shows parts for items that dont have parts

LP#1466990: Detailed search results shows parts for items that dont have parts

The part_label variable needs to be reset between iterations. The variable was still set
and bled from copy to copy. Testing this will require a search with details turned on.
The search result set needs to include a bib that has copies with parts (with at least
one copy invisible) and another bib that has copies without parts.

Signed-off-by: blake <blake@mobiusconsortium.org>
Signed-off-by: Ben Shum <ben@evergreener.net>