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=1006710Signed-off-by: NLiuji (Jeremy) <jeremy.liu@huawei.com>
(cherry picked from commit ef5d51d4)

Conflicts:
	src/conf/domain_conf.c