Fix depth of stack when underscript depth is negative.
Fix depth of stack when underscript depth is negative.

thomasjm/MathJax,GerHobbelt/MathJax,thomasjm/MathJax,thomasjm/MathJax,thomasjm/MathJax,dpvc/MathJax,jinntrance/mathjax,GerHobbelt/MathJax,wikimedia/MathJax,jinntrance/mathjax,thomasjm/MathJax