Fixed open bus behavior

[100thCoin]

Currently, reading from open bus on an Everdrive N8 Pro (from address $4020 through $7FFF) always returns the high byte of the address bus. This can be quite easily fooled by running LDA $50FF, X where X is non-zero. Let's say X is 1. The LDA instruction will perform a dummy read on address $5000, where the value $50 will be read (from the high byte operand), and then the second read will be on address $5100. With the current logic, this would return $51, which is incorrect, as the data bus should still be unchanged from when the high byte operand was read. Similarly, a DMC DMA would change the data bus value, which wouldn't be visible on an everdrive outside of the $4000 through $401F range.

This change that I made simply reads the data bus value back instead of the high byte of the address bus. I've made a test for open bus behavior in my accuracy ROM, AccuracyCoin. Here's the results before and after this change:

Before:

Error code 3 is: "Indexed addressing crossing a page boundary should not update the data bus to the new high byte value."

After: