tor-cell: Assert data length in Data cells

This commit adds a `debug_assert!` macro into the `new_unchecked()`
function of the Data cell. Beside this, it also fixes a misleading
comment regarding that limit.

crates/tor-cell/src/relaycell/msg.rs

@@ -399,8 +399,9 @@ impl Data {
 
     /// Construct a new data cell from a provided vector of bytes.
     ///
-    /// The vector _must_ have fewer than [`Data::MAXLEN`] bytes.
+    /// The vector _must_ not have more than [`Data::MAXLEN`] bytes.
     fn new_unchecked(body: Vec<u8>) -> Self {
+        debug_assert!(body.len() <= Data::MAXLEN);
         Data { body }
     }
 }