std-shims: six Read for &[u8]

common/std-shims/src/io.rs

@@ -53,7 +53,7 @@ mod shims {
 
   impl Read for &[u8] {
     fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
-      let mut read = 0;
+      let mut read = buf.len();
       if self.len() < buf.len() {
         read = self.len();
       }