Fixed Array.prototype.slice() with slow "this" argument.

Fixed Array.prototype.slice() with slow "this" argument.

Previously, when "this" argument was not a fast array, but the "deleted" array
was a fast array, the "deleted" array may be left in uninitialized state if
"this" argument had gaps.

This fix is to ensure that "deleted" is properly initialized.

This fixes #485 issue on Github.