utilities::range_check: Correct range_check expression

Previously, we were multiplying the expression by 0, which led it
to always evaluate to true.

src/circuit/gadget/utilities.rs

@@ -123,10 +123,9 @@ pub fn bitrange_subset<F: FieldExt + PrimeFieldBits>(field_elem: F, bitrange: Ra
 /// Check that an expression is in the small range [0..range),
 /// i.e. 0 ≤ word < range.
 pub fn range_check<F: FieldExt>(word: Expression<F>, range: usize) -> Expression<F> {
-    (0..range)
-        .map(|i| Expression::Constant(F::from_u64(i as u64)))
-        .reduce(|acc, i| acc * (word.clone() - i))
-        .expect("range > 0")
+    (1..range).fold(word.clone(), |acc, i| {
+        acc * (word.clone() - Expression::Constant(F::from_u64(i as u64)))
+    })
 }
 
 #[cfg(test)]