selinux: do not cancel a policy conversion that never started

In the Linux kernel, the following vulnerability has been resolved: selinux: do not cancel a policy conversion that never started sel_write_load() calls selinux_policy_cancel() when sel_make_policy_nodes() fails, and that helper dereferences the outgoing policy to cancel its sidtab conversion. On the first policy load there is no outgoing policy: security_load_policy() returns early for that case, before it converts anything, and state->policy is still NULL. A first load that fails while building the selinuxfs tree therefore takes a NULL dereference in selinux_policy_cancel(), reached from a write(2) to /sys/fs/selinux/load. Skip the cancel when there is no old policy, mirroring the check security_load_policy() already makes before it converts.