agent: Fix segv running in --server mode

* agent/command.c (start_command_handler): Do not write to
CLIENT_CREDS after an error.

assuan_get_peercred is special insofar that it returns a pointer into
CTX. Writing data via this pointer should never be done.

• Fixes-commit: 28aa6890588cc108639951bb4bef03ac17743046
• Signed-off-by: Werner Koch <wk@gnupg.org>