curl_xml.c: avoid using uninitalized variable in error message

Thanks to @trtrmitya for reporting this. Fixes GH#507
interface.c: FreeBSD-10 support

Quoting @trtrmitya in github issue #506 : "[...] it is broken on
FreeBSD-10, in which getifaddrs() returns not only link level stats for
a particular interface, but also entries for each IP configured on that
interface. As a result if_submit() is called several times for each
interface, which results in incorrect data being logged.

I am attaching a patch which fixes a problem on FreeBSD (9/10), but it
should work for every *BSD because [...] the getifaddrs implementation
first appeared in BSDi BSD/OS."

Many thanks to @trtrmitya for providing the patch !
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.