agent: Fix possible release of unitialize var in a genkey error case.
* agent/command.c (cmd_genkey): Initialize 'value'.
(cherry picked from commit 2bdc4b6ed97770ed15ec6c5afa02c2e44568a3bc)
- GnuPG-bug-id: T4222
- Signed-off-by: Werner Koch <wk@gnupg.org>