io: Avoid Avoid marking bytes as uninit in BufReader::peek

Author: a1phyr

library/std/src/io/buffered/bufreader/buffer.rs

@@ -124,7 +124,6 @@ impl Buffer {
     pub fn backshift(&mut self) {
         self.buf.copy_within(self.pos.., 0);
         self.initialized -= self.pos;
-        self.filled -= self.pos;
         self.pos = 0;
     }