selinux: do not crash if policy becomes unavailable after reload

https://bugzilla.redhat.com/show_bug.cgi?id=1944171

Thread 1 (Thread 0x7fb5f0341b80 (LWP 1974)):
№0  selabel_lookup_common (rec=0x0, translating=0, key=0x55f616ac4750 "/run/user/1000/systemd/units/invocation:systemd-tmpfiles-clean.service", type=40960) at label.c:167

'rec' is the handle that we passed.

№1  0x00007fb5f13ae87f in selabel_lookup_raw (rec=<optimized out>, con=con@entry=0x7fffef307380, key=key@entry=0x55f616ac4750 "/run/user/1000/systemd/units/invocation:systemd-tmpfiles-clean.service", type=type@entry=40960) at label.c:256
        lr = <optimized out>

'rec' is passed through as is to selabel_lookup_common().

№2  0x00007fb5f1561b2d in selinux_create_file_prepare_abspath (abspath=0x55f616ac4750 "/run/user/1000/systemd/units/invocation:systemd-tmpfiles-clean.service", mode=40960) at ../src/basic/selinux-util.c:368
        filecon = 0x0
        r = <optimized out>
        __PRETTY_FUNCTION__ = "selinux_create_file_prepare_abspath"
        __func__ = "selinux_create_file_prepare_abspath"

№3  0x00007fb5f1561ec3 in mac_selinux_create_file_prepare (path=<optimized out>, mode=40960) at ../src/basic/selinux-util.c:431
        r = 0
        abspath = 0x55f616ac4750 "/run/user/1000/systemd/units/invocation:systemd-tmpfiles-clean.service"
        __PRETTY_FUNCTION__ = "mac_selinux_create_file_prepare"

We checked label_hnd != NULL, but then we apparently called
avc_netlink_check_nb(), which reset label_hnd. Yay for global state!

№4  0x00007fb5f1549950 in symlink_atomic_label (from=0x55f6169d8b50 "69a8dcf7a7ac46b29306f2fddbed3edc", to=0x55f616ab8380 "/run/user/1000/systemd/units/invocation:systemd-tmpfiles-clean.service") at ../src/basic/label.c:55
        r = <optimized out>
        __PRETTY_FUNCTION__ = "symlink_atomic_label"

In the logs:

Mar 29 14:48:44 fedorapad.home systemd[1974]: selinux: avc:  received policyload notice (seqno=2)
Mar 29 14:48:44 fedorapad.home systemd[1974]: Failed to initialize SELinux labeling handle: No such file or directory
Mar 29 14:48:44 fedorapad.home systemd[1974]: selinux: avc:  received policyload notice (seqno=3)
Mar 29 14:48:44 fedorapad.home systemd[1974]: selinux: avc:  received setenforce notice (enforcing=0)