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.

crypto/krb5/src/lib/gssapi/spnego/negoex_util.c

@@ -253,6 +253,10 @@ parse_nego_message(OM_uint32 *minor, struct k5input *in,
     offset = k5_input_get_uint32_le(in);
     count = k5_input_get_uint16_le(in);
     p = vector_base(offset, count, EXTENSION_LENGTH, msg_base, msg_len);
+    if (p == NULL) {
+        *minor = ERR_NEGOEX_INVALID_MESSAGE_SIZE;
+        return GSS_S_DEFECTIVE_TOKEN;
+    }
     for (i = 0; i < count; i++) {
         extension_type = load_32_le(p + i * EXTENSION_LENGTH);
         if (extension_type & EXTENSION_FLAG_CRITICAL) {
@@ -391,7 +395,8 @@ parse_message(OM_uint32 *minor, spnego_gss_ctx_id_t ctx, struct k5input *in,
     msg_len = k5_input_get_uint32_le(in);
     conv_id = k5_input_get_bytes(in, GUID_LENGTH);
 
-    if (in->status || msg_len > token_remaining || header_len > msg_len) {
+    if (in->status || msg_len > token_remaining ||
+        header_len < (size_t)(in->ptr - msg_base) || header_len > msg_len) {
         *minor = ERR_NEGOEX_INVALID_MESSAGE_SIZE;
         return GSS_S_DEFECTIVE_TOKEN;
     }