Fix EXG instruction exception

[callsop]

• fix uninitialized xfreg16 array elements 6&7.
• fix access violation reading address zero on undefined register in 6809 mode, now returns FFFF when reading invalid register.
• yet to find out which program caused this but its clearly possible.

[ejaquay]

Someday we should test this on a real coco3. Maybe returning FFFF is not what it does but that is a minor point. As far as which program I imagine this is most likely to occur when attempting to run something written for 6309.