io: Avoid marking buffer as uninit when copying to BufWriter

Author: a1phyr

library/std/src/io/copy.rs

@@ -249,7 +249,6 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
                 }
             } else {
                 self.flush_buf()?;
-                init = 0;
             }
         }
     }