Return Ok directly

Clippy emits warning:

	warning: passing a unit value to a function

Just return `Ok(())` after calling `fill_bytes`.

src/key.rs

@@ -512,7 +512,8 @@ mod test {
                 self.0 -= 1;
             }
             fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
-                Ok(self.fill_bytes(dest))
+                self.fill_bytes(dest);
+                Ok(())
             }
         }