Fix Key.sign by setting buffer length

The truncate() method doesn't do anything if the "len" is greater
than the current length, which is the case here.
So instead, use the Vec.set_len method.

Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>

Author: puiterwijk

src/api.rs

@@ -935,7 +935,9 @@ impl Key {
         let mut buffer = Vec::with_capacity(support.max_sig_size as usize);
         let write_buffer = buffer.get_backing_buffer();
         let sz = keyctl_pkey_sign(self.id, &info, data, write_buffer)?;
-        buffer.truncate(sz);
+        unsafe {
+            buffer.set_len(sz);
+        }
         Ok(buffer)
     }