Fix bug in graph_union() with filtered graphs

6 jobs for master in 184 minutes and 10 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #2277
amd64
job_clang_amd64

03:04:10

passed #2281
amd64
job_clang_amd64_nosh

02:55:27

passed #2279
amd64
job_clang_py2_amd64

02:56:19

passed #2276
amd64
job_gcc_amd64

02:17:58

passed #2280
amd64
job_gcc_amd64_nosh

02:12:24

passed #2278
amd64
job_gcc_py2_amd64

02:15:33