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

Coverity 184942

src/blockchain_utilities/blockchain_usage.cpp

@@ -243,10 +243,17 @@ int main(int argc, char* argv[])
     counts[out.second.size()]++;
     total++;
   }
-  for (const auto &c: counts)
+  if (total > 0)
   {
-    float percent = 100.f * c.second / total;
-    MINFO(std::to_string(c.second) << " outputs used " << c.first << " times (" << percent << "%)");
+    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");