libidn2 -- roundtrip check vulnerability

CVE list:

GNU libidn2 before 2.2.0 fails to perform the roundtrip checks
specified in RFC3490 Section 4.2 when converting A-labels to U-labels.
This makes it possible in some circumstances for one domain to
impersonate another. By creating a malicious domain that matches a
target domain except for the inclusion of certain punycoded Unicode
characters (that would be discarded when converted first to a Unicode
label and then back to an ASCII label), arbitrary domains can be
impersonated.