curl_xml plugin: Fix potential dereference of NULL pointer.

I managed to get collectd to segfault in a couple of places while
playing with it a bit. The first is in the curl_xml module when the
XPATH expression doesn't quite match the input. The crash occurs on line
407 when instance_node->nodeTab[0] is dereferenced. At this point, all
members of instance_node are 0, so dereferencing the array isn't a good
idea. This patch fixes the problem, although I'm not sure if this
particular case actually deserves its own error message.

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