Agon locks up when POS and/or VPOS is used with VDU 31

[tonedef71]

I presumed this was an issue with the VDP, but it was suggested that the issue might need to be fixed in BBC BASIC, so I am re-submitting the issue here.

Any of the following lines of code in BASIC or BASIC ADL will cause the Agon to lock-up:

10 VDU 31, POS, VPOS
20 VDU 31, 0, VPOS
30 VDU 31, POS, 0

The VDP should handle the evaluation of POS and VPOS properly so that the proper X and Y coordinates are passed to VDU 31.

[breakintoprogram]

As it is the same bug, and to avoid repeating myself, comments are being updated in the ADL version here:

breakintoprogram/agon-bbc-basic-adl#12