conf: explicitly initialize 'cpumask' variable

Build with clang fails with:

  CC       conf/libvirt_conf_la-domain_conf.lo
  conf/domain_conf.c:13377:9: error: variable 'cpumask' is used
  uninitialized whenever 'if' condition is true
  [-Werror,-Wsometimes-uninitialized]
      if (!(tmp = virXMLPropString(node, "cpuset"))) {
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and many other similar errors regarding the 'cpuset' variable.

Fix by explicitly initializing it with NULL.

src/conf/domain_conf.c

@@ -13342,7 +13342,7 @@ virDomainIOThreadPinDefParseXML(xmlNodePtr node,
 {
     int ret = -1;
     virDomainIOThreadIDDefPtr iothrid;
-    virBitmapPtr cpumask;
+    virBitmapPtr cpumask = NULL;
     xmlNodePtr oldnode = ctxt->node;
     unsigned int iothreadid;
     char *tmp = NULL;