pvcalls-front: fix potential null dereference

 static checker warning:
    drivers/xen/pvcalls-front.c:373 alloc_active_ring()
    error: we previously assumed 'map->active.ring' could be null
           (see line 357)

drivers/xen/pvcalls-front.c
    351 static int alloc_active_ring(struct sock_mapping *map)
    352 {
    353     void *bytes;
    354
    355     map->active.ring = (struct pvcalls_data_intf *)
    356         get_zeroed_page(GFP_KERNEL);
    357     if (!map->active.ring)
                    ^^^^^^^^^^^^^^^^^
Check

    358         goto out;
    359
    360     map->active.ring->ring_order = PVCALLS_RING_ORDER;
    361     bytes = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
    362                     PVCALLS_RING_ORDER);
    363     if (!bytes)
    364         goto out;
    365
    366     map->active.data.in = bytes;
    367     map->active.data.out = bytes +
    368         XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
    369
    370     return 0;
    371
    372 out:
--> 373     free_active_ring(map);
                                 ^^^
Add null check on map->active.ring before dereferencing it to avoid
any NULL pointer dereferences.

Fixes: 9f51c05d ("pvcalls-front: Avoid get_free_pages(GFP_KERNEL) under spinlock")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Juergen Gross <jgross@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Dan Carpenter <dan.carpenter@oracle.com>
CC: xen-devel@lists.xenproject.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>