graph_draw(): Avoid drawing selected vertices, when none exist

This fixes issue #551
6 jobs for master in 177 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #3955
amd64
job_clang_amd64

02:57:33

failed #3959
amd64
job_clang_amd64_nosh

02:57:31

failed #3957
amd64
job_clang_py2_amd64

02:57:33

failed #3954
amd64
job_gcc_amd64

02:57:12

failed #3958
amd64
job_gcc_amd64_nosh

02:57:12

failed #3956
amd64
job_gcc_py2_amd64

02:57:13

 
Name Stage Failure
failed
job_clang_py2_amd64 Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/3/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/3/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/3/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/3/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1
failed
job_clang_amd64 Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/1/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/1/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/1/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/1/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1
failed
job_clang_amd64_nosh Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/5/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/5/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/5/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/5/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1
failed
job_gcc_amd64_nosh Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/4/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/4/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/4/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/4/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1
failed
job_gcc_py2_amd64 Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/2/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/2/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/2/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/2/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1
failed
job_gcc_amd64 Test
make[4]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool/src/graph/inference'
make[3]: *** [Makefile:809: all-recursive] Error 1
make[3]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool/src/graph'
make[2]: *** [Makefile:422: all-recursive] Error 1
make[2]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool/src'
make[1]: *** [Makefile:594: all-recursive] Error 1
make[1]: Leaving directory '/var/lib/gitlab-runner/builds/c098b1de/0/count0/graph-tool'
make: *** [Makefile:481: all] Error 2
ERROR: Job failed: exit status 1