ipconfig: Possible to have double unref

ipconfig: Possible to have double unref

Because we do not check that origin pointer is not this ipconfig,
it is possible that we might try to unref the ipconfig one time
too many.
Something like this was seen in the log file:

connmand[19208]: src/ipconfig.c:enable_ipv6()
connmand[19208]: src/ipconfig.c:__connman_ipconfig_unref() ipconfig 0x47502f0 refcount 0
connmand[19208]: src/ipconfig.c:__connman_ipconfig_disable() ipconfig 0x47502f0
connmand[19208]: src/ipconfig.c:disable_ipv6()
connmand[19208]: src/ipconfig.c:__connman_ipconfig_unref() ipconfig 0x47502f0 refcount -1