hw/net/xen_nic.c: Free 'netdev->txs' when map 'netdev->rxs' fails

When map 'netdev->rxs' fails, need free the original resource, or will
cause resource leak.
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/xen_nic.c

@@ -370,11 +370,16 @@ static int net_connect(struct XenDevice *xendev)
                                           netdev->xendev.dom,
                                           netdev->tx_ring_ref,
                                           PROT_READ | PROT_WRITE);
+    if (!netdev->txs) {
+        return -1;
+    }
     netdev->rxs = xc_gnttab_map_grant_ref(netdev->xendev.gnttabdev,
                                           netdev->xendev.dom,
                                           netdev->rx_ring_ref,
                                           PROT_READ | PROT_WRITE);
-    if (!netdev->txs || !netdev->rxs) {
+    if (!netdev->rxs) {
+        xc_gnttab_munmap(netdev->xendev.gnttabdev, netdev->txs, 1);
+        netdev->txs = NULL;
         return -1;
     }
     BACK_RING_INIT(&netdev->tx_ring, netdev->txs, XC_PAGE_SIZE);