Polymorph traps, when they polymorph the player, should always choose a form considered to be “playable”. I.e. it must have greater than 0 speed, and must have hands or be fast enough to run from threats or be well-defended enough to sustain damage until transforming back.

However, the polymorph should then be permanent - you will not re-form with your original HP when your polyform is killed, you merely die.