Fix a bug

Before:
$ salt-call pip.list tinypng
salt-dev:

git+https://github.com/saltstack/salt.git@77e758c2e8142d92298aa26db08ed6
af02506038
tinypng:
    1.2.0

After:
$ salt-call pip.list tinypng
tinypng:
    1.2.0
Merge pull request #5562 from zhangcheng/Fix_pip.list

Fix a bug of pip.list