port select fails if invalid exists

[nhojpatrick (John Patrick)]

I've just done a weekly upgrade and a prompt appeared about python. So I did as suggested, I then played around with other python versions and discovered select doesn't clean up if active versions are uninstalled and then you try and install a new version as active.

$ sudo port select --list python3
Available versions for python3:
	none (active)
	python35
$ ls -l /opt/local/etc/select/python3/
total 32
-rw-r--r--+ 1 root  admin  145 12 Sep 13:27 base
lrwxr-xr-x  1 root  admin    8 21 Aug  2015 current -> python34
-rw-r--r--+ 1 root  admin   20 12 Sep 13:27 none
-rw-r--r--+ 1 root  admin  154 18 Oct 20:11 python35
$ sudo port select --set python3 python35
Selecting 'python35' for 'python3' failed: symlink: /opt/local/etc/select/python3/current -> python35: file already exists

I would attempt a port but no idea where to start and already trying to fix the githut portgroup.

I would guess this could happen with every port select. If an invalid symlink exists and you try and set a new one it should delete the existing symlink and create it as specified.

[ryandesign (Ryan Carsten Schmidt)]

Duplicate of #31702.