When the textual representation of an IPv6 nxt_sockaddr_t was being
generated, a crash would occur if the address had a full IPv6 form:
f607:7403:1e4b:6c66:33b2:843f:2517:da27
This was caused by a variable that tracks the location of a
collapsed group ("::") that was not set to a sane default. When
the address was generated, a group would be inserted when
it was not necessary, thus causing an overflow.
This closes#481 issue on GitHub.
7d2bc04e39