conf: Don't free uninitialized pointer

This causes libvirtd to crash when both <boot dev='...'/> and <boot
order='...'/> are used in one domain XML. Introduced by
5fa3d775a9

src/conf/domain_conf.c

@@ -6430,7 +6430,8 @@ virDomainDefParseBootXML(xmlXPathContextPtr ctxt,
 {
     xmlNodePtr *nodes = NULL;
     int i, n;
-    char *bootstr, *useserial;
+    char *bootstr;
+    char *useserial = NULL;
     int ret = -1;
     unsigned long deviceBoot, serialPorts;