fixed command receiving code

source/main.cpp

@@ -49,9 +49,11 @@ int main()
 	while (true) {
         if (usbd.active()) {
             // Expect to receive a byte command 'packet'.
-            if (char cmd[3]; usbd.read(&cmd, 3) > 0) {
+            if (char cmd[3]; usbd.read(&cmd, 1) > 0) {
                 switch (cmd[0]) {
                 case 'r': // Read in analog signal
+                    if (usbd.read(&cmd[1], 2) < 2)
+                        break;
                     if (auto count = std::min(static_cast<unsigned int>(cmd[1] | (cmd[2] << 8)), adc_samples.size()); count > 0) {
                         adc.getSamples(&adc_samples[0], count);
                         usbd.write(adc_samples.data(), count * sizeof(adcsample_t));