txpool: Properly bail out when outputs_amount == inputs_amount

Previously, when outputs_amount == inputs_amount, the "m_overspend" property
was set, whereas "m_fee_too_low" would have been the correct property to set.

This is unlikely to ever occur and just something I've noticed while reading
through the code.

src/cryptonote_core/tx_pool.cpp

@@ -151,13 +151,20 @@ namespace cryptonote
       }
 
       uint64_t outputs_amount = get_outs_money_amount(tx);
-      if(outputs_amount >= inputs_amount)
+      if(outputs_amount > inputs_amount)
       {
         LOG_PRINT_L1("transaction use more money then it has: use " << print_money(outputs_amount) << ", have " << print_money(inputs_amount));
         tvc.m_verifivation_failed = true;
         tvc.m_overspend = true;
         return false;
       }
+      else if(outputs_amount == inputs_amount)
+      {
+        LOG_PRINT_L1("transaction fee is zero: outputs_amount == inputs_amount, rejecting.");
+        tvc.m_verifivation_failed = true;
+        tvc.m_fee_too_low = true;
+        return false;
+      }
 
       fee = inputs_amount - outputs_amount;
     }