idna: fix OOB read in punycode decoder

libuv was vulnerable to out-of-bounds reads in the uv__idna_toascii()
function which is used to convert strings to ASCII. This is called by
the DNS resolution function and can lead to information disclosures or
crashes.

b7466e31e4
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=990561
https://nodejs.org/en/blog/vulnerability/july-2021-security-releases/

Signed-off-by: Hirokazu MORIKAWA <morikw2@gmail.com>