net/slirp: fix memory leak

commit b412eb6 introduce 'cmd:' target for guestfwd,
and fwd don't be used in this scenario, and will leak
memory in true branch with 'cmd:'. Let's allocate memory
for fwd variable just in else statement.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

net/slirp.c

@@ -643,17 +643,16 @@ static int slirp_guestfwd(SlirpState *s, const char *config_str,
         goto fail_syntax;
     }
 
-    fwd = g_malloc(sizeof(struct GuestFwd));
     snprintf(buf, sizeof(buf), "guestfwd.tcp.%d", port);
 
     if ((strlen(p) > 4) && !strncmp(p, "cmd:", 4)) {
         if (slirp_add_exec(s->slirp, 0, &p[4], &server, port) < 0) {
             error_report("conflicting/invalid host:port in guest forwarding "
                          "rule '%s'", config_str);
-            g_free(fwd);
             return -1;
         }
     } else {
+        fwd = g_malloc(sizeof(struct GuestFwd));
         fwd->hd = qemu_chr_new(buf, p, NULL);
         if (!fwd->hd) {
             error_report("could not open guest forwarding device '%s'", buf);