Prevent occasional SEGV when freeing list

[Hoops]

The maxminddb API seems to need a pointer to the first element when passing to MMDB_free_entry_data_list. Observed occasional SEGVs under heavy load without this fix.

We just save a the head of the list before dumping and pass that to MMDB_free_entry_data_list.

Note this is the same way they do it in the C API:

https://github.com/maxmind/libmaxminddb/blob/fc183662e85cb1b252c2f0272dc41f79e52e50fa/src/maxminddb.c#L730.

CC #8, #9

[anjia0532]

LGTM

[anjia0532]

thanks for your job