parse_ipv6addr: add some bounds checking on indexes

[lucab]

Capturing my comment from #294 (comment).

parse_ipv6addr does a bit of index juggling when decompressing input, and it may benefit from some bound checking to ensure no index underflow/overflow will induce a panic there at runtime.

This exists in current master here, but it is likely going to be changed by #294.

[SimonSapin]

Closing as this doesn’t seem very actionable. Please comment here or open a new issue if there is a specific test case that panics where it shouldn’t.