scd: More fix for Curve25519 prefix handling.
* scd/app-openpgp.c (do_decipher): Handle trancated cipher text. Also fix xfree bug introduced.
In old format with no prefix, cipher text can be trancated when it
is parsed as MPI. Recover the value adding back zeros.
- Fixes-commit: 11b2691eddc42e91651e4f95dd2731255a3e9211