Fix contract parameter max length

[limemloh]

Purpose

The max smart contract parameter length was changed in protocol version 5 and onwards.

Changes

Change the max parameter length to the new limit of 65535 bytes

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.