Merge branch 'fix-dangling-ref' into 'master'

Fix danging reference bug caused by halfedge().from_vertex().

See merge request !62
9 jobs for master in 5 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #75788
VS2015
VS2015-32-bit

00:01:16

passed #75787
VS2015
VS2015-64-bit

00:01:19

passed #75786
VS2017
VS2017-32-bit

00:01:09

passed #75785
VS2015
VS2017-64-bit

00:01:16

passed #75782
Linux
clang-c++11

00:04:11

passed #75784
Linux
cppcheck

00:00:22

passed #75789
Linux
doc

00:00:19

passed #75781
Linux
gcc-c++11

00:03:43

passed #75783
Apple
macos-c++11

00:01:37