fixes #93 stop endless loop when node parent is null

[edanp]

added checking if node is null in the while condition

[andreskrey]

I'm always wary of merging this type of fixes. Adding safe checks around the code is just a patch, where the real problem is hiding somewhere else. The engine should never reach the HTML tag while scanning parents.

I'll keep this pull open as a reminder and explore the real issue over the weekend.

[edanp]

I completely agree with you about this being a patch more than a fix for the real bug, but not merging it means you leave the library with a bug that cause endless loop.
Don't you think it's a priority to release a fix for this as a minor update and keep the bigger unknown bug as a todo for when you or anyone else have time?