bc: forbid negative array index

[mknos]

• Negative array index is being caught when accessing a value (p instruction), but is not caught when assigning a value (P instruction)
• Adapt existing guard code from 'p' instruction
• For consistency with undefined-function error, replace YYERROR call with "last INSTR"
• Only the 2nd-from-top ope_stack element is needed; fetch it with splice instead of pop+push dance for $value

%perl bc -d # before patch
ary[-1]
instruction: N, 1
instruction: m_
instruction: p, ary
Non-integer index -1 for array

ary[-1] = 123
instruction: N, 1
instruction: m_
instruction: N, 123
instruction: P, ary
instruction: =P
wat

[briandfoy]

changes: catch negative index in array assignment