Fix RoundRobin crash with zero output gate

If RoundRobin module has no output gate, it crashes due to division by
zero. In this case, the "right" behavior should be just dropping
packets.

Also, this commit replaces the modulo(%) operation with a simple if
branch. It saves about 22 cycles per batch or packet, depending on the
operation mode of RoundRobin.

Author: sangjinhan

core/modules/round_robin.cc

@@ -101,15 +101,24 @@ CommandResponse RoundRobin::CommandSetGates(
 void RoundRobin::ProcessBatch(bess::PacketBatch *batch) {
   gate_idx_t out_gates[bess::PacketBatch::kMaxBurst];
 
+  if (ngates_ <= 0) {
+    bess::Packet::Free(batch);
+    return;
+  }
+
   if (per_packet_) {
     for (int i = 0; i < batch->cnt(); i++) {
       out_gates[i] = gates_[current_gate_];
-      current_gate_ = (current_gate_ + 1) % ngates_;
+      if (++current_gate_ >= ngates_) {
+        current_gate_ = 0;
+      }
     }
     RunSplit(out_gates, batch);
   } else {
     gate_idx_t gate = gates_[current_gate_];
-    current_gate_ = (current_gate_ + 1) % ngates_;
+    if (++current_gate_ >= ngates_) {
+      current_gate_ = 0;
+    }
     RunChooseModule(gate, batch);
   }
 }