fix [#33689] Crash with decimate modifier

in fact a more general bug in BM_face_create_ngon() where edges containing the same vertex multiple times were not accounted for.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53521 954f8c5b-7b00-dc11-b283-0030488c597c