curl_json, curl_xml: Fix unitialized variable

The variable url is used unintialized here. The code used to be the same
in both plugins, but diverged in 19808b44. The solution applied there
does not work correctly as the effective URL can only be queried after
performing the request. Instead, just use the original request URL.

Signed-off-by: Florian Forster <octo@collectd.org>