krb5: Fix two NegoEx parsing vulnerabilities

Bring in upstream commit 2e75f0d93 fixing two CVEs. Upstream commit
log is:

In parse_nego_message(), check the result of the second call to
vector_base() before dereferencing it. In parse_message(), check for
a short header_len to prevent an integer underflow when calculating
the remaining message length.

Reported by Cem Onat Karagun.

CVE-2026-40355:

In MIT krb5 release 1.18 and later, if an application calls
gss_accept_sec_context() on a system with a NegoEx mechanism
registered in /etc/gss/mech, an unauthenticated remote attacker can
trigger a null pointer dereference, causing the process to terminate.

CVE-2026-40356:

In MIT krb5 release 1.18 and later, if an application calls
gss_accept_sec_context() on a system with a NegoEx mechanism
registered in /etc/gss/mech, an unauthenticated remote attacker can
trigger a read overrun of up to 52 bytes, possibly causing the process
to terminate. Exfiltration of the bytes read does not appear
possible.