While a pane of glass travelling at tornado speeds can decapitate a person, a person cannot be decapitated by a pane of glass "thrown" by a tornado. Mostly because tornadoes don't throw things.
Anything loose can and will be picked up by a tornado, but the tornado will keep them, not throw them away. Things moved by tornadoes are dropped when it dissipates, not thrown out.
So the only way to be hit by a pane of glass is to be in the funnel of the tornado itself, in which case decapitation is the least of your worries.
Also, there's little chance of a sheet of glass being horizontal in a tornado because there is almost no surface area for the air to push against, as opposed to the pane vertically where the air has a greater area to act upon.