xen/pvcalls: fix null pointer dereference on map->sock

Currently if map is null then a potential null pointer deference
occurs when calling sock_release on map->sock.  I believe the
actual intention was to call sock_release on sock instead. Fix
this.

Fixes: 5db4d286 ("xen/pvcalls: implement connect command")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NJuergen Gross <jgross@suse.com>

drivers/xen/pvcalls-back.c

@@ -416,7 +416,7 @@ static int pvcalls_back_connect(struct xenbus_device *dev,
 					sock);
 	if (!map) {
 		ret = -EFAULT;
-		sock_release(map->sock);
+		sock_release(sock);
 	}
 
 out: