pushd on current directory

Created by: zen3ger

Reproduction:

zen3ger:~/Projects/ion# pushd .
. /home/zen3ger/Projects/ion
zen3ger:~/Projects/ion# cd ../
ion: failed to set current dir to : No such file or directory (os error 2)
zen3ger:~/Projects/ion# popd
/home/zen3ger/Projects/ion
zen3ger:~/Projects/ion# cd ../
zen3ger:~/Projects/ion# pwd
/home/zen3ger/Projects

Expected behavior: I should be able to push . on the dir stack and back up a directory. After popd the prompt should show the current directory path.

Actual behavior: ion fails to execute cd ../ on . directory. After removing . from the dir stack with popd the prompt freezez and shows the directory name where popd got executed.

Build information: ion 1.0.0-alpha (x86_64-unknown-linux-gnu) rev 18b6f5eb