s390x/pci: return directly if create zpci failed

In the case that zpci is automatically created, we did not return
immediately on failure, which would lead to NULL pointer dereferencing.
Let's fix it.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/s390-pci-bus.c

@@ -649,6 +649,7 @@ static void s390_pcihost_hot_plug(HotplugHandler *hotplug_dev,
             pbdev = s390_pci_device_new(dev->id);
             if (!pbdev) {
                 error_setg(errp, "create zpci device failed");
+                return;
             }
         }