pr: memory leak on infinite data streams

[Devel08]

GNU pr doesn't leak memory, on uutils it happens on /dev/zero, /dev/full and /dev/random

reproduce:

$ cargo run pr /dev/random
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
     Running `target/debug/coreutils pr /dev/random`