storage: Fix a problem which will cause libvirtd crashed

If pool xml has no definition for "port", then "Segmentation fault"
happens when jumping to "cleanup:" to do "VIR_FREE(port)", as "port"
was not initialized in this situation.

* src/conf/storage_conf.c

src/conf/storage_conf.c

@@ -396,7 +396,7 @@ virStoragePoolDefParseSource(xmlXPathContextPtr ctxt,
     char *authType = NULL;
     int nsource, i;
     virStoragePoolOptionsPtr options;
-    char *port;
+    char *port = NULL;
 
     relnode = ctxt->node;
     ctxt->node = node;