Fixed Array.prototype.sort() when array is changed while sorting.

Fixed Array.prototype.sort() when array is changed while sorting.

Previously, the fast-path check did not take into account the fact that
the flat array may be resized as a side effect of the array's values
evaluation.

In addition, the slow_path fix ensures that "this" array is repopulated
again even if the array was resized.

This fixes #594 issue on Github.