Fix RoundRobin crash with zero output gate

[sangjinhan]

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.

[codecov]

Codecov Report

Merging #623 into master will decrease coverage by 0.02%.
The diff coverage is 71.42%.

@@            Coverage Diff            @@
##           master    #623      +/-   ##
=========================================
- Coverage   69.32%   69.3%   -0.03%     
=========================================
  Files         204     204              
  Lines       13068   13072       +4     
=========================================
- Hits         9060    9059       -1     
- Misses       4008    4013       +5

Impacted Files	Coverage Δ
core/modules/round_robin.cc	52.94% <71.42%> (-0.26%)	⬇️
core/modules/port_out.cc	80.95% <0%> (-2.39%)	⬇️
core/drivers/unix_socket.cc	80.91% <0%> (-0.77%)	⬇️
core/modules/flowgen.cc	73.7% <0%> (-0.38%)	⬇️

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[melvinw]

Thanks!