network: L2TP fix crash

[ssahani]

=220358== Invalid read of size 8
==220358==    at 0x452F05: l2tp_session_free (l2tp-tunnel.c:46)
==220358==    by 0x456926: l2tp_tunnel_done (l2tp-tunnel.c:725)
==220358==    by 0x43CF4D: netdev_free (netdev.c:205)
==220358==    by 0x43D045: netdev_unref (netdev.c:210)
==220358==    by 0x4198B7: manager_free (networkd-manager.c:1877)
==220358==    by 0x40D0B3: manager_freep (networkd-manager.h:105)
==220358==    by 0x40DE1C: run (networkd.c:21)
==220358==    by 0x40DE75: main (networkd.c:130)
==220358==  Address 0x5c035d0 is 0 bytes inside a block of size 40 free'd
==220358==    at 0x483A9F5: free (vg_replace_malloc.c:538)
==220358==    by 0x452F87: l2tp_session_free (l2tp-tunnel.c:57)
==220358==    by 0x456857: netdev_l2tp_tunnel_verify (l2tp-tunnel.c:710)
==220358==    by 0x440947: netdev_load_one (netdev.c:738)
==220358==    by 0x441222: netdev_load (netdev.c:851)
==220358==    by 0x419C50: manager_load_config (networkd-manager.c:1934)
==220358==    by 0x40D7BE: run (networkd.c:87)
==220358==    by 0x40DE75: main (networkd.c:130)
==220358==  Block was alloc'd at
==220358==    at 0x4839809: malloc (vg_replace_malloc.c:307)
==220358==    by 0x452A76: malloc_multiply (alloc-util.h:96)
==220358==    by 0x4531E6: l2tp_session_new_static (l2tp-tunnel.c:82)
==220358==    by 0x455C01: config_parse_l2tp_session_id (l2tp-tunnel.c:535)
==220358==    by 0x48E6D72: next_assignment (conf-parser.c:133)
==220358==    by 0x48E77A3: parse_line (conf-parser.c:271)
==220358==    by 0x48E7E4F: config_parse (conf-parser.c:396)
==220358==    by 0x48E80E5: config_parse_many_files (conf-parser.c:453)
==220358==    by 0x48E8490: config_parse_many (conf-parser.c:512)
==220358==    by 0x44089C: netdev_load_one (netdev.c:729)
==220358==    by 0x441222: netdev_load (netdev.c:851)
==220358==    by 0x419C50: manager_load_config (networkd-manager.c:1934)

closes #15968

[keszybz]

centos ci (arch in kvm) failed in TEST-13-NSPAWN-SMOKE, unrelated.