Reject odd number of hexits rather than ignoring the lone trailing hexit

implementations/rust/preserves/src/hex.rs

@@ -36,7 +36,11 @@ impl HexParser {
                     },
             }
         }
-        Some(result)
+        if buf_full {
+            None // odd number of hexits
+        } else {
+            Some(result)
+        }
     }
 }