agent: Fix stack buffer overflow when using gpgsm and KEM
* agent/pkdecrypt.c (ecc_kem_decrypt): Check sessionkey_len before calling gcry_cipher_decrypt. Always close decryption handle.
This bug was indroduced with 2.5.13 (2025-10-22). It affects only
this branch.
- GnuPG-bug-id: T8044