charon-tkm: Validate DH public key to fix potential buffer overflow

charon-tkm: Validate DH public key to fix potential buffer overflow

Seems this was forgotten in the referenced commit and actually could lead
to a buffer overflow.  Since charon-tkm is untrusted this isn't that
much of an issue but could at least be easily exploited for a DoS attack
as DH public values are set when handling IKE_SA_INIT requests.

Fixes: 0356089d0f94 ("diffie-hellman: Verify public DH values in backends")
Fixes: CVE-2023-41913