move 16 byte check to fields

blackbox_solver/src/barretenberg/wasm/scalar_mul.rs

@@ -25,24 +25,24 @@ impl ScalarMul for Barretenberg {
         let low_bytes = low.to_be_bytes();
         let high_bytes = high.to_be_bytes();
 
-        let low_16_bytes = low_bytes[16..32].to_vec();
-        let high_16_bytes = high_bytes[16..32].to_vec();
-
-        let mut bytes = high_16_bytes.to_vec();
-        bytes.extend_from_slice(&low_16_bytes);
-
         let two_pow_128 = BigUint::from(2u128).pow(128);
-        if BigUint::from_bytes_be(&low_16_bytes) >= two_pow_128 {
+        if BigUint::from_bytes_be(&low_bytes) >= two_pow_128 {
             return Err(Error::FromFeature(FeatureError::InvalidGrumpkinScalarLimb {
-                limb_as_hex: hex::encode(low_16_bytes),
+                limb_as_hex: hex::encode(low_bytes),
             }));
         }
-        if BigUint::from_bytes_be(&high_16_bytes) >= two_pow_128 {
+        if BigUint::from_bytes_be(&high_bytes) >= two_pow_128 {
             return Err(Error::FromFeature(FeatureError::InvalidGrumpkinScalarLimb {
-                limb_as_hex: hex::encode(high_16_bytes),
+                limb_as_hex: hex::encode(high_bytes),
             }));
         }
 
+        let low_16_bytes = low_bytes[16..32].to_vec();
+        let high_16_bytes = high_bytes[16..32].to_vec();
+
+        let mut bytes = high_16_bytes.to_vec();
+        bytes.extend_from_slice(&low_16_bytes);
+
         // Check if this is smaller than the grumpkin modulus
         let grumpkin_integer = BigUint::from_bytes_be(&bytes);
         let grumpkin_modulus = BigUint::from_bytes_be(&[