selinux_child: Fix crash with initialized key

The semanage_seuser_key_t can be NULL in done section
in case of issues with initializing semanage handle or failure
with creating transaction.

The semanage_seuser_key_free is not NULL tolerant therefore its better
to prevent dereference of NULL pointer.

  #0  semanage_seuser_key_free_internal (key=0x0) at seuser_record.c:83
  #1  0x000055c1f8a687ed in sss_set_seuser (login_name=0x55c1f9ff8a50 "user4_1",
          seuser_name=0x55c1f9ff8960 "staff_u", mls=<optimized out>)
          at src/util/sss_semanage.c:344
  #2  0x000055c1f8a67eea in sc_set_seuser (mls=0x55c1f9ff89d0 "s0-s0:c0.c1023",
          seuser_name=0x55c1f9ff8960 "staff_u",
          login_name=0x55c1f9ff8a50 "user4_1")
          at src/providers/ipa/selinux_child.c:162
  #3  main (argc=<optimized out>, argv=<optimized out>)
          at src/providers/ipa/selinux_child.c:334

Merges: https://pagure.io/SSSD/sssd/pull-request/3732

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

src/util/sss_semanage.c

@@ -341,7 +341,9 @@ int sss_set_seuser(const char *login_name, const char *seuser_name,
 
     ret = EOK;
 done:
-    semanage_seuser_key_free(key);
+    if (key != NULL) {
+        semanage_seuser_key_free(key);
+    }
     sss_semanage_close(handle);
     return ret;
 }