silcclient: handle command timeout error correctly

silcclient: handle command timeout error correctly

If command which can return a list of entries such as the LIST command
timeouts the cmd->status may be something other than SILC_STATUS_OK,
ie. _LIST_END for example which then means ERROR_CALLBACK won't update
the SILC_STATUS_ERR_TIMEOUT correctly to the cmd context.  This can
crash the application handling the command reply.  Clear the cmd->status
in case the command timedout.