Fix result count display when posts_per_page is set to -1

When your posts_per_page is set to -1 the display is output as `Showing 1–-1 of 15 results` when displaying all items when it would be better to show `Showing all 15 results`
Merge pull request #3596 from ninnypants/fix_product_result_count

Fix result count display when posts_per_page is set to -1 @coenjacobs .14