agent: Slightly change structure of cmd_readkey.

* agent/command.c (cmd_readkey): Avoid a leave label in the middle of
the code.  Remove the special return.

This helps to get better debug output.

The set_error macro which is used by parse_keygrip merely sets the
error code into the Assuan context. It is thus no problem anymore to
call leave_cmd after having used set_error. This might havve been
diffferent in the past.

• Signed-off-by: Werner Koch <wk@gnupg.org>