fix big-endian merkleization for UintN arrays

UintN arrays were incorrectly merkleized on big-endian. This was fixed
by making sure to use the correct buffer to store the final chunk.

Author: etan-status

beacon_chain/ssz/merkleization.nim

@@ -464,7 +464,7 @@ func chunkedHashTreeRootForBasicTypes[T](merkleizer: var SszMerkleizerImpl,
     if remainingValues > 0:
       var lastChunk: array[bytesPerChunk, byte]
       for i in 0 ..< remainingValues:
-        chunk.writeBytesLE(i * sizeof(T), arr[writtenValues + i])
+        lastChunk.writeBytesLE(i * sizeof(T), arr[writtenValues + i])
       merkleizer.addChunk lastChunk
 
   getFinalHash(merkleizer)