Be more careful on command exit.

If the callback returned zero, there's nothing to do.
If it returned positive, we have to send out the result
buffer and must free it afterwards. Sending the buffer may
well lead to an error condition, so always check the return
value of the corresponding call to send_buffer(). Finally,
it the command handler returned a negative value, we have
to send an error message to the client which might give rise
to another error (if sending the errror message failed). In this
case, ignore the send errror and return the previous error value.