Fix validation of overly-long IPv6 addresses.

The inet/cidr types sometimes failed to reject IPv6 inputs with too many
colon-separated fields, instead translating them to '::/0'.  This is the
result of a thinko in the original ISC code that seems to be as yet
unreported elsewhere.  Per bug #14198 from Stefan Kaltenbrunner.

Report: <20160616182222.5798.959@wrigleys.postgresql.org>

src/backend/utils/adt/inet_net_pton.c

@@ -496,7 +496,7 @@ inet_cidr_pton_ipv6(const char *src, u_char *dst, size_t size)
 			else if (*src == '\0')
 				goto enoent;
 			if (tp + NS_INT16SZ > endp)
-				return (0);
+				goto enoent;
 			*tp++ = (u_char) (val >> 8) & 0xff;
 			*tp++ = (u_char) val & 0xff;
 			saw_xdigit = 0;