fix(machine): ensure the pointer is valid even when cLen is zero

Author: mpolitzer

pkg/emulator/machine.go

@@ -333,7 +333,13 @@ func (m *Machine) ReceiveCmioRequest() (uint8, uint16, []byte, error) {
 		if err != nil {
 			return
 		}
+
+		// guard against empty `data`
+		if cLen == 0 {
+			cLen = 1
+		}
 		data = make([]byte, cLen)
+
 		err = newError(C.cm_receive_cmio_request(
 			m.ptr,
 			&cCmd,