gpg: Fix modifying signature data by pk_verify for Ed25519.
* g10/pkglue.c (pk_verify): When fixing R and S, make sure those are copies.
Fixing-commit: 0a5a854510fda6e6990938a3fca424df868fe676
Also avoid clearing the error by the S code of a failed mpi_print of
R.
- GnuPG-bug-id: T7426
- Signed-off-by: Werner Koch <wk@gnupg.org>