Some dhcpv6 fixes (#5071)

Some dhcpv6 fixes (#5071)

* dhcpv6: Fix potential memory leak

* dhcpv6: Fix typo in default virtual server

* dhcpv6: Don't crash if 'interface' is empty.

It fixes:

Mon Jun 19 23:40:31 2023: Bootstrapping proto_dhcpv4 "dhcp.dhcpv4"
Mon Jun 19 23:40:31 2023: Bootstrapping proto_dhcpv6 "dhcpv6.all_dhcp_servers_and_relays"
Process 73144 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x00000001a68c853c libsystem_platform.dylib`_platform_strcmp + 12
libsystem_platform.dylib`:
->  0x1a68c853c <+12>: ldrb   w5, [x1], #0x1
    0x1a68c8540 <+16>: subs   x3, x4, x5
    0x1a68c8544 <+20>: ccmp   w4, #0x0, #0x4, eq
    0x1a68c8548 <+24>: b.ne   0x1a68c8530               ; <+0>
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x00000001a68c853c libsystem_platform.dylib`_platform_strcmp + 12
    frame #1: 0x0000000100368d60 libfreeradius-util.dylib`fr_interface_to_ethernet(interface=0x0000000000000000, ethernet=0x0000600003e041b0) at inet.c:1570:7
    frame #2: 0x0000000100ed7c7c proto_dhcpv6_udp.dylib`mod_bootstrap(mctx=0x000000016fdfdd60) at proto_dhcpv6_udp.c:564:9
    frame #3: 0x00000001002aecbc libfreeradius-io.dylib`mod_bootstrap(mctx=0x000000016fdfe100) at master.c:2610:41
    frame #4: 0x0000000100b85260 proto_dhcpv6.dylib`mod_bootstrap(mctx=0x000000016fdfe890) at proto_dhcpv6.c:470:9
    frame #5: 0x0000000100619bd8 libfreeradius-server.dylib`module_bootstrap(mi=0x0000000102214890) at module.c:778:7
    frame #6: 0x0000000100619d0c libfreeradius-server.dylib`modules_bootstrap(ml=0x0000600002fa85e0) at module.c:811:7
    frame #7: 0x000000010066c9b0 libfreeradius-server.dylib`virtual_servers_bootstrap(config=0x0000000100ff2c30) at virtual_servers.c:1301:6
    frame #8: 0x00000001005ce87c libfreeradius-server.dylib`server_init(cs=0x0000000100ff2c30) at base.c:68:6
    frame #9: 0x00000001000043a0 radiusd`main(argc=7, argv=0x000000016fdfecb8) at radiusd.c:788:6