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 clang-c++11 #75731
Linux

00:02:35

passed cppcheck #75733
Linux

00:04:20

passed doc #75748
Linux

00:00:20

passed gcc-c++11 #75730
Linux

00:04:25

passed macos-c++11 #75732
Apple

00:01:39

passed VS2015-32-bit #75737
VS2015

00:01:16

passed VS2015-64-bit #75736
VS2015

00:01:21

passed VS2017-32-bit #75735
VS2017

00:01:09

passed VS2017-64-bit #75734
VS2015

00:01:17

failed doc #75738
Linux

00:00:09