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

Author: a1phyr

Diff for: std/src/io/buffered/bufreader/buffer.rs

@@ -123,7 +123,6 @@ impl Buffer {
     /// Remove bytes that have already been read from the buffer.
     pub fn backshift(&mut self) {
         self.buf.copy_within(self.pos.., 0);
-        self.initialized -= self.pos;
         self.filled -= self.pos;
         self.pos = 0;
     }