You're very lucky that it wouldn't shift into a too short combination. The chain is vastly stronger than any other part of your bike, and if it succeeds in looping two sprockets it isn't long enough (or even tries) you could face expensive consequences.
If you're lucky you'll bend an axle as the chain tries to pull the cassette forward. It might also rip off the derailleur if the cage cannot come above the straight line lower loop, beyond those minor annoyances, it could tear off the right dropout, or buckle your chainrings or bend the right crank.
In summary a too-short chain can destroy your bike.
You have a simple way to avoid this. Replace the chain now, today, before you ride this bike again. It probably isn't as urgent as I make it sound, but consider the potential expense if your luck turns sour.
Also don't let misplaced concern about chain/cassette issues deter you from doing what needs to be done. Chains are replaced every day, and most of us can go through 3-5 chains before the cassette needs replacing. if it turns out your cassette is worn, you'll need to replace that, but a worn cassette would need replacing soon enough anyway.
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