Uploader: really restrict circular dependencies check to current site

When checking for circular dependencies, there was a way for circles in
dependencies outside this update site's purview to result in an error.

Therefore, let's really stop when we encounter a dependency that is not
on the current update site.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix compiler warning

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>