found cause of cgen gas-test failures in list-ref

[Alan Lehotsky <qsmgmt@earthlink.net>]

I found the source of the problem.  And since I've seen mail from  
others who seemed confused by similar problems I'll
share my methodology and results....

I had a register file with only 8 entries, indexed by an external- 
keyword with 8 entries;  but the instruction field holding these  
values was 6 bits (allowing potentially 64 registers to be physically  
addressed.

This caused the keyword operand generation in gas-test.scm ~line 40 to  
try and index into the define-keyword table beyond the end.

I found this with a combination of the magical

       (logit 1 .......)

function to print out trace messages and eventually by replacing the  
failing call

           (list-ref test-cases n)

by

          (list-maybe-ref test-cases n)

and instrumenting the else-clause of the (if (< n (length l)) to print  
out the last good index value.

No real idea how best to fix this in the source -- I'm pretty rusty as  
a lisp programmer (although I have used the IBM 7094 responsible for  
the CAR and CDR acronyms....)  If I had to guess, it would be to avoid  
map-ing elements that are short....

I decided to work around it by not allowing a 6 bit instruction field.