TypedArray: coerce length argument once on buffer construction

TypedArray: coerce length argument once on buffer construction

Previously, njs_typed_array_alloc() passed the length argument through
njs_value_to_index() twice, before and after the detached-buffer check,
and kept only the second result.  The argument's valueOf() was invoked
twice, which is observable.

The fix is to remove the second conversion.