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

implementations/rust/preserves/src/hex.rs

@@ -36,9 +36,13 @@ impl HexParser {
                     },
             }
         }
+        if buf_full {
+            None // odd number of hexits
+        } else {
             Some(result)
         }
     }
+}
 
 impl HexFormatter {
     pub fn encode(&self, bs: &[u8]) -> String {