Fix `ToLength` in polyfill to handle a `NaN` length properly.
Fix `ToLength` in polyfill to handle a `NaN` length properly.

Almost fixes #7.

tc39/proposal-string-pad-start-end,tc39/proposal-string-pad-start-end,ljharb/proposal-string-pad-left-right,ljharb/proposal-string-pad-left-right,tc39/proposal-string-pad-left-right,tc39/proposal-string-pad-left-right