`IP_ADDR_V6: unbound variable` on host w/o IPv6

[Haarolean]

Hi! First of all, thanks for this nice tool.

I get this error on a machine without IPv6:

./porkbun-ddns.sh: line 193: IP_ADDR_V6: unbound variable

It would be nice to be able to make it work even without v6 :)

[zhuwenbing]

It has been fixed. Thank you for your valuable comments.

[Haarolean]

@zhuwenbing thank you very much!