Handle properly a case where VIRTUAL_ENV is unset
If `$VIRTUAL_ENV` is not set but `.venv` exists (when auto-switching
occurs), the following happens:
```
basename: missing operand
Try 'basename --help' for more information.
Switching virtualenv: xxxxx [Python 2.7.12]
```
To avoid this, we add a condition to check whether VIRTUAL_ENV is set.
Diff
1diff --git a/autoswitch_virtualenv.plugin.zsh b/autoswitch_virtualenv.plugin.zsh
2index bb80f3c4502cb8786e0234ea66ec0dbc29bba01e..effa6d695163b61af167bdaaa19dfe8b45cd9108 100644
3--- a/autoswitch_virtualenv.plugin.zsh
4+++ b/autoswitch_virtualenv.plugin.zsh
5@@ -3,7 +3,7 @@ function precmd() {
6 }
7 8 function maybeworkon() {
9- if [ "$1" != "$(basename $VIRTUAL_ENV)" ]; then
10+ if [[ -z "$VIRTUAL_ENV" || "$1" != "$(basename $VIRTUAL_ENV)" ]]; then
11 if [ -z "$AUTOSWITCH_SILENT" ]; then
12 printf "Switching virtualenv: %s " $1
13 fi