Fix "-1" array accesses in CommentsParser.

[bmeurer]

Similar to the fixes in babel/babel#6580 and
babel/babel#6581, accesses of the form

stack[stack.length - 1];

when stack can be an empty array are pretty bad for performance.
In this case it also breaks the type safety, since the function
last<T> is declared to only return values of type T, but
occasionally also returns undefined now, since the stack parameters
passed to it never contain a property "-1" and neither do the
Object.prototype or the Array.prototype.

This is a non-breaking performance fix, which adds proper checking
to ensure that last is only invoked on non-empty arrays.

[hzoo]

Nice, thanks @bmeurer! (and turning off comments in the parser saves a lot as well 😂), since honestly just need the license comments for Babel and should drop everything else