Release shared reference on zero truncate (#842)

Co-authored-by: JSap0914 <[email protected]>

src/bytes.rs

@@ -567,7 +567,9 @@ impl Bytes {
     /// ```
     #[inline]
     pub fn truncate(&mut self, len: usize) {
-        if len < self.len {
+        if len == 0 {
+            drop(mem::replace(self, Bytes::new_empty_with_ptr(self.ptr)));
+        } else if len < self.len {
             // The Vec "promotable" vtables do not store the capacity,
             // so we cannot truncate while using this repr. We *have* to
             // promote using `split_off` so the capacity can be stored.