Add recursion limit for ASN.1 indefinite lengths

The libkrb5 ASN.1 decoder supports BER indefinite lengths.  It
computes the tag length using recursion; the lack of a recursion limit
allows an attacker to overrun the stack and cause the process to
crash.  Reported by Demi Obenour.

CVE-2020-28196:

In MIT krb5 releases 1.11 and later, an unauthenticated attacker can
cause a denial of service for any client or server to which it can
send an ASN.1-encoded Kerberos message of sufficient length.

ticket: 8959 (new)
tags: pullup
target_version: 1.18-next
target_version: 1.17-next