Existing proxies check isn't working

First, in my case it always generates the proxies.

Running with -vvv shows that it tries the ffprobe -v error -select_streams v:0 ... command which for me outputs 260 and exit code 0.

I briefly looked at the code but it's a bit complex to parse that part. I'd suggest to use Python list comprehension (map(), compress()... are great but harder to reads and frowned upon nowadays by many).