I would refer to other rules in the PHB regarding reverting to your original form. Both Wild Shape and Polymorph return you to your HP total from your original form before the transformation when your transformed form HP reach 0. Other transformative spells have similar wording. I would rule this is the case here, as well.
Given that the effect occurs on a roll of 41 or 42, even if this isn't the accepted ruling by the community as a whole, it's okay to have fun with the game. This option (using the Wild Shape/Polymorph rule for HP determination) is a more lighthearted version of the effect than the one the community agrees upon (use your current HP and when you hit 0 you revert and are dying).
I'd hate to have a PC die because he turned into a potted plant because of an Easter egg hidden in the game rules for comic relief (the reference, if you've missed it, is the number 42 resulting in the player spontaneously turning into a potted plant -- see the Hitchhiker's Guide to the Galaxy for more information). Then again, that could make it even funnier: "Hey, remember the time I turned into a potted plant and that orc smashed me into tiny bits and then I started dying?"
Of course, at any table, it is okay to use either ruling. When in doubt, talk to your DM.