libidn: add fix for CVE-2017-14062

libidn: add fix for CVE-2017-14062

Add upstream patch fixing CVE-2017-14062:

Integer overflow in the decode_digit function in puny_decode.c in
Libidn2 before 2.0.4 allows remote attackers to cause a denial of
service or possibly have unspecified other impact.

This issue also affects libidn.

Unfortunately, the patch also triggers reconf of the documentation
subdirectory, since lib/punycode.c is listed in GDOC_SRC that is defined
in doc/Makefile.am. Add autoreconf to handle that.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>