Check in strictly::ChunkedBytes::bytes_vectored

Add a debug_assert checking the invariant that the staging buffer
does not exceed the chunk size limit.

Author: mzabaluev

src/strictly.rs

@@ -223,6 +223,7 @@ impl Buf for ChunkedBytes {
     /// entries filled.
     #[inline]
     fn bytes_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
+        debug_assert!(self.inner.staging_len() <= self.inner.chunk_size());
         self.inner.bytes_vectored(dst)
     }