conf: Avoid false positive of uninitialized variable use

conf: Avoid false positive of uninitialized variable use

GCC 4.8.0+ whines about variable "new" being uninitialized since
commit 73bfac0e7182a3abd. This is a false positive as the
xmlFreeNode(new) statement can be only reached if new was actually
allocated successfully.

  CC       conf/libvirt_conf_la-domain_conf.lo
  conf/domain_conf.c: In function 'virDomainDefSetMetadata':
  conf/domain_conf.c:18650:24: error: 'new' may be used uninitialized in this function [-Werror=maybe-uninitialized]
               xmlFreeNode(new);

Reported independently by John Ferlan and Michal Privoznik.