Fix handling of variables inside comprehensions

[CoffeeTableEspresso]

Currently, the comprehension assumes it is the only thing on the stack, which may not be correct:

const x = [ 1, 2, 3, 4, 45, [60] ]

echo [ .a, .b, .c ] + [ x for x <- x ]
# incorrectly prints [60, 1, 2, 3, 4, 45, [60]]