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

We should not return references to members of possible xvalues.
Handle is lightweight enough that copying is not a problem.
10 jobs for fix-dangling-ref in 4 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #75737
VS2015
VS2015-32-bit

00:01:16

passed #75736
VS2015
VS2015-64-bit

00:01:21

passed #75735
VS2017
VS2017-32-bit

00:01:09

passed #75734
VS2015
VS2017-64-bit

00:01:17

passed #75731
Linux
clang-c++11

00:02:35

passed #75733
Linux
cppcheck

00:04:20

passed #75748
Linux
doc

00:00:20

passed #75730
Linux
gcc-c++11

00:04:25

passed #75732
Apple
macos-c++11

00:01:39

failed #75738
Linux
doc

00:00:09