heimdal: CVE-2022-41916: Check for overflow in _gsskrb5_get_mech()

heimdal: CVE-2022-41916: Check for overflow in _gsskrb5_get_mech()

Apply upstream 22749e918 to fix a buffer overflow.

Upstream notes:

    If len_len is equal to total_len - 1 (i.e. the input consists only of a
    0x60 byte and a length), the expression 'total_len - 1 - len_len - 1',
    used as the 'len' parameter to der_get_length(), will overflow to
    SIZE_MAX. Then der_get_length() will proceed to read, unconstrained,
    whatever data follows in memory. Add a check to ensure that doesn't
    happen

This is similar to samba CVE-2022-3437.

Reported by:	emaste
Security:	CVE-2022-41916
Obtained from:	upstream 22749e918
MFC after:	1 week