Fix stdio try_read() behavior. Closes #473. (#475)

libs/stdio/src/lib.rs

@@ -251,10 +251,14 @@ impl<'a> StdioReadGuard<'a> {
         let mut locked_ring_buf = self.guard.lock();
 
         // Keep reading if we have empty space in the output buffer
-        // and available byte in the ring buffer.
-        while let (Some(buf_elem), Some(queue_elem)) = (buf_iter.next(), locked_ring_buf.queue.pop_front()) {
-            *buf_elem = queue_elem;
-            cnt += 1;
+        // and available byte(s) in the ring buffer.
+        while let Some(buf_entry) = buf_iter.next() {
+            if let Some(queue_elem) = locked_ring_buf.queue.pop_front() {
+                *buf_entry = queue_elem;
+                cnt += 1;
+            } else {
+                break;
+            }
         }
 
         return Ok(cnt);