NES: Fixed GetInternalOpenBus returning external bus value by mistake.

[zdg-kinlon]

Summary

Fix OpenBusHandler::GetInternalOpenBus() returning external bus value

Description

Fixed a bug in OpenBusHandler where GetInternalOpenBus() incorrectly returned _externalOpenBus instead of _internalOpenBus.

Technical Details

• Root Cause: The method returned _externalOpenBus when it should return _internalOpenBus.
• Impact: While $ 4015 reads may appear unaffected in simple cases (since GetInternalOpenBus() is called before updating the bus), the bug breaks correctness whenever the internal and external bus values differ — such as after a $ 4015 read or in advanced mappers/debuggers that rely on accurate internal bus state.
• Hardware Behavior: Reading $ 4015 does not drive the external bus; only the CPU's internal bus is updated. Thus, internal and external open bus values must be tracked separately.

APU#Status_($4015)
Note: The $ 4015 bit 5 uses the internal open bus value from before the read, which is correctly captured in the current call order. The bug primarily affects other consumers of GetInternalOpenBus() or future code relying on bus state separation.