Fix io::BufReader uses of read_buf

Author: a1phyr

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

@@ -349,7 +349,7 @@ impl<R: ?Sized + Read> Read for BufReader<R> {
         let prev = cursor.written();
 
         let mut rem = self.fill_buf()?;
-        rem.read_buf(cursor.reborrow())?;
+        rem.read_buf(cursor.reborrow())?; // actually never fails
 
         self.consume(cursor.written() - prev); //slice impl of read_buf known to never unfill buf
 

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

@@ -133,11 +133,13 @@ impl Buffer {
                 buf.set_init(self.initialized);
             }
 
-            reader.read_buf(buf.unfilled())?;
+            let result = reader.read_buf(buf.unfilled());
 
             self.pos = 0;
             self.filled = buf.len();
             self.initialized = buf.init_len();
+
+            result?;
         }
         Ok(self.buffer())
     }