chore: deduplicate array access

Author: TomAFrench

src/sha256.nr

@@ -163,13 +163,15 @@ pub(crate) fn process_full_blocks<let N: u32>(
 
         blocks[num_blocks] = new_msg_block;
     }
+    
     // verify the 0 padding is correct for the last block
+    let final_block = blocks[first_partially_filled_block_index]; 
     verify_msg_block_zeros(
-        blocks[first_partially_filled_block_index],
+        final_block,
         message_size % BLOCK_SIZE,
         INT_BLOCK_SIZE,
     );
-    (states[first_partially_filled_block_index], blocks[first_partially_filled_block_index])
+    (states[first_partially_filled_block_index], final_block)
 }
 
 // Take `BLOCK_SIZE` number of bytes from `msg` starting at `msg_start` and pack them into a `MSG_BLOCK`.