CSE Machine: Stops without error message for array access

[alsonleej]

1[2]; (LHS is not array)
[1][2]; (array access out of bounds)
currently makes the CSE machine get stuck without any visible error.

[martin-henz]

[1][2];
should return undefined. See
https://docs.sourceacademy.org/source_3.pdf
Access of an array with an array index to which no prior assignment has been made on the array
returns undefined.