Fix parse error for z_sender_sockaddr

[krawthekrow]

Allows z_sender_sockaddr to be ZASCII, following the C version (https://github.com/zephyr-im/zephyr/blob/master/lib/ZParseNot.c#L251). I'm not sure if there's updated documentation for this, but Zephyr returns it in ZASCII when I do a subscribe.

[davidben]

Thanks!

Do you mind adding a quick test to cover this path? It's been a while since I've seen any of this code, but it looks like TestDecodeNotice in notice_test.go has a zcode test. You could just add the corresponding zascii one.

Bonus stuff If you feel motivated to, but no need to bother if not: In hindsight, 8.8.8.8 was a really dumb choice of test input on my part, since it doesn't catch endianness. Probably, like, any other IP address would have been better. :-) Though I guess sampleNotice/sampleRawNotice have another IP in there. I probably also should have tested IPv6.

[krawthekrow]

Sure! I've added the test.