In snmpe_writecb, don't call snmpe_prepare_read() when there's still

In snmpe_writecb, don't call snmpe_prepare_read() when there's still
data left inside sm_data. If there's an incomplete packet left in the
buffer it will be called from snmpe_tryparse, if there's a complete
packet left we can end up with new events from the tcp socket, which the
tcp subsystem isn't prepared to handle.

OK tb@