race condition in ring buffer

[nerdralph]

UartByteCount is modified in the UART1 ISR and in main. If the UART1 interrupt occurs between lines 590 and 598, the received byte will be dropped.
https://github.com/Blinkinlabs/ch554_sdcc/blob/master/examples/usb_device_cdc/main.c#L598

I don't see a simple fix. I think it would be best to replace the ring buffer code with a thread-safe version. I recently published a tiny ring buffer that would be a good fit.
https://github.com/nerdralph/RingBuffer