USB: Only swap incoming buffers for non-ZLP's

[sandeepmistry]

I noticed Serial.available() was stuck returning 0 when I sent 1033 bytes from the Serial Monitor in the IDE from macOS. Sketch snipping:

  do {
    if (SerialUSB.available()) {
      char c = SerialUSB.read();

       SerialUSB.print(c);

       publicCert += (char)c;
    }
  } while (!publicCert.endsWith("-----END CERTIFICATE-----"));

It turns out a ZLP was being received, and the USB code was swapping incoming buffers, when it didn't need to and this confused the code in available() which never swaps back to the other buffer.

This change prevents the swap in the USB endpoint ISR handler when a ZLP is received. NOTE: the call to release(); is needed, this is why I didn't use the if (last0 == 0) { return; } style.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b150_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request USB: Only swap incoming buffers for non-ZLP's #231
5. Select one of the boards under SAMD Pull Request USB: Only swap incoming buffers for non-ZLP's #231 in Tools->Board menu
6. Compile/Upload as usual

[cmaglie]

👍 👍 👍

[sandeepmistry]

@cmaglie could you please create a new 1.6.16 milestone for this?