#1973 Handle redirected G2 URLs for tags, including g2_itemId passed

[was: Don't jump to /tag_name/foo if we're targeting a specific g2_ItemId]

This check was in the wrong place; it works for a URL of the form:

https://example.com/gallery3/g2/map?path=tag/foo

but it fails if given:

https://example.com/gallery3/g2/map?path=tag/foo&g2_itemId=1234

More precisely, I expect the second to take me to a specific image (assuming, of course, that 1234 has a mapping in g2_map), but it immediately jumps to the tag name. With this fix, it respects the item if one is found (I missed this in https://github.com/gallery/gallery3/pull/86)