The effect of Multiply by Zero overrides any other effect that sets a creature's base power and toughness, as well as any characteristic-defining abilities that set base power and toughness (such as that of Fblthp, Knows the Way, another card in Reality Fracture). However, if any other effects are increasing that creature's toughness, such as a +1/+1 counter or an effect that gave the creature +1/+1 until end of turn, those will still apply, and in that case, the creature will not die due to having 0 toughness.