blockchain_usage: don't divide by 0 when there is nothing to process

Coverity 184942

src/blockchain_utilities/blockchain_usage.cpp

@@ -243,11 +243,18 @@ int main(int argc, char* argv[])
     counts[out.second.size()]++;
     total++;
   }
+  if (total > 0)
+  {
     for (const auto &c: counts)
     {
       float percent = 100.f * c.second / total;
       MINFO(std::to_string(c.second) << " outputs used " << c.first << " times (" << percent << "%)");
     }
+  }
+  else
+  {
+    MINFO("No outputs to process");
+  }
 
   LOG_PRINT_L0("Blockchain usage exported OK");
   return 0;