UDP.available() calculates the buffer incorrectly.

[baender]

I was making some tests with the Arduino MKR 1500 and a Python UDP server running on my machine (network via 1NCE VPN).

Sending data to the server did work, but when the server responded (returning message in all caps), the number of bytes available was around 4.294.967.290 (0xFFFF FFFA). So I figured that the subtraction used in NBUdp.cpp line 262 is reversed. After changing it I always got the correct number of bytes.
The solution is an educated guess, perhaps someone with more in-depth knowledge wants to have a look into that. However, I will post a Pull Request.