blockchain: initialize pow to ff..ff

as a safety to reject if it somehow does not get initialised

src/cryptonote_core/blockchain.cpp

@@ -1694,7 +1694,8 @@ bool Blockchain::handle_alternative_block(const block& b, const crypto::hash& id
     // Check the block's hash against the difficulty target for its alt chain
     difficulty_type current_diff = get_next_difficulty_for_alternative_chain(alt_chain, bei);
     CHECK_AND_ASSERT_MES(current_diff, false, "!!!!!!! DIFFICULTY OVERHEAD !!!!!!!");
-    crypto::hash proof_of_work = null_hash;
+    crypto::hash proof_of_work;
+    memset(proof_of_work.data, 0xff, sizeof(proof_of_work.data));
     if (b.major_version >= RX_BLOCK_VERSION)
     {
       crypto::hash seedhash = null_hash;
@@ -3700,7 +3701,8 @@ leave:
 
   TIME_MEASURE_START(longhash_calculating_time);
 
-  crypto::hash proof_of_work = null_hash;
+  crypto::hash proof_of_work;
+  memset(proof_of_work.data, 0xff, sizeof(proof_of_work.data));
 
   // Formerly the code below contained an if loop with the following condition
   // !m_checkpoints.is_in_checkpoint_zone(get_current_blockchain_height())