service: Handle NULL pointer in __connman_service_set_{domain|host}name

516af0fd1586 ("service: Sanitize input for hostname and domainname")
address the shutdown path:

connmand[12534]: ++++++++ backtrace ++++++++
connmand[12534]: #0  0x7fefba7195c0 in /lib64/libc.so.6
connmand[12534]: #1  0x7fefbab8caa4 in /lib64/libglib-2.0.so.0
connmand[12534]: #2  0x44c600 in __connman_service_set_domainname() at src/service.c:2664
connmand[12534]: #3  0x462d16 in apply_dhcp_invalidate_on_network() at src/dhcp.c:110
connmand[12534]: #4  0x463c87 in dhcp_free() at src/dhcp.c:70
connmand[12534]: #5  0x442bd8 in set_disconnected() at src/network.c:1009
connmand[12534]: #6  0x442e10 in network_remove() at src/network.c:1120
connmand[12534]: #7  0x442e77 in connman_network_driver_unregister() at src/network.c:1211
connmand[12534]: #8  0x41cf88 in ethernet_exit() at plugins/ethernet.c:462
connmand[12534]: #9  0x43f1d9 in __connman_plugin_cleanup() at src/plugin.c:202
connmand[12534]: #10 0x4113f4 in main() at src/main.c:874
connmand[12534]: #11 0x7fefba705413 in /lib64/libc.so.6
1 file changed