After freeing the bitmap pointer, it must set the pointer to NULL.
This will avoid any other use of the freed memory of the bitmap pointer.
https://bugzilla.redhat.com/show_bug.cgi?id=1006710
Signed-off-by: Liuji (Jeremy) <jeremy.liu@huawei.com>
(cherry picked from commit ef5d51d491356f1f4287aa3a8b908b183b6dd9aa)