xen/pvcalls: check for xenbus_read() errors

Smatch complains that "len" is uninitialized if xenbus_read() fails so
let's add some error handling.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-front.c

@@ -1128,6 +1128,8 @@ static int pvcalls_front_probe(struct xenbus_device *dev,
 	}
 
 	versions = xenbus_read(XBT_NIL, dev->otherend, "versions", &len);
+	if (IS_ERR(versions))
+		return PTR_ERR(versions);
 	if (!len)
 		return -EINVAL;
 	if (strcmp(versions, "1")) {