Revert "curl_xml.c: avoid using uninitalized variable in error message"

This reverts commit bbbf37d55a3959951604c4be482e9a705a0f86d9.

A different fix for this problem was commited to the collectd-5.4 branch
(8327ee64) and conflicts with this patch. Also, initializing url the
same way as 8327ee64 does it, to avoid a merge conflict.