boykov_kolmogorov_max_flow(): Fix problem with parallel reciprocal edges

This fixes issue #598
5 jobs for master in 408 minutes and 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
canceled #4089
amd64
job_clang_amd64

canceled #4084
amd64
job_clang_amd64_nosh

06:48:54

canceled #4081
amd64
job_gcc_amd64

06:48:56

canceled #4083
amd64
job_gcc_amd64_nosh

06:48:52

canceled #4082
amd64
job_clang_amd64

06:48:45