Avoid second getSingleTrueBit call

packages/beacon-node/src/chain/validation/attestation.ts

@@ -326,6 +326,7 @@ async function validateAttestationNoSignatureCheck(
   }
 
   let aggregationBits: BitArray | null = null;
+  let committeeValidatorIndex: number | null = null;
   if (!isForkPostElectra(fork)) {
     // [REJECT] The attestation is unaggregated -- that is, it has exactly one participating validator
     // (len([bit for bit in attestation.aggregation_bits if bit]) == 1, i.e. exactly 1 bit is set).
@@ -345,6 +346,7 @@ async function validateAttestationNoSignatureCheck(
         code: AttestationErrorCode.NOT_EXACTLY_ONE_AGGREGATION_BIT_SET,
       });
     }
+    committeeValidatorIndex = bitIndex;
   }
 
   let committeeValidatorIndices: Uint32Array;
@@ -399,16 +401,13 @@ async function validateAttestationNoSignatureCheck(
   }
 
   let validatorIndex: number;
-  let committeeValidatorIndex: number;
 
   if (!isForkPostElectra(fork)) {
     // The validity of aggregation bits are already checked above
     assert.notNull(aggregationBits);
-    const bitIndex = aggregationBits.getSingleTrueBit();
-    assert.notNull(bitIndex);
+    assert.notNull(committeeValidatorIndex);
 
-    validatorIndex = committeeValidatorIndices[bitIndex];
-    committeeValidatorIndex = bitIndex;
+    validatorIndex = committeeValidatorIndices[committeeValidatorIndex];
     // [REJECT] The number of aggregation bits matches the committee size
     // -- i.e. len(attestation.aggregation_bits) == len(get_beacon_committee(state, data.slot, data.index)).
     // > TODO: Is this necessary? Lighthouse does not do this check.