cli: fix use of uninitialized variable in "notmuch reply"

notmuch_show_params_t params is only initialized partially in
notmuch_reply_command(). The only field that is used uninitialized is
params.decrypt. It is usually non-zero, making "notmuch reply" on encrypted
messages work by coincidence.

Initialize params properly, and set params.decrypt as needed.

Signed-off-by: Jani Nikula <jani@nikula.org>