umlConnectTapDevice: initialize tapfd variable

There is possibility to jump to 'cleanup' label without tapfd variable
being initialized. In the label, VIR_FORCE_CLOSE(tapfd) is called which
can have fatal consequences.

src/uml/uml_conf.c

@@ -109,7 +109,7 @@ umlConnectTapDevice(virConnectPtr conn,
                     const char *bridge)
 {
     bool template_ifname = false;
-    int tapfd;
+    int tapfd = -1;
 
     if (!net->ifname ||
         STRPREFIX(net->ifname, VIR_NET_GENERATED_PREFIX) ||