fixes case where name is omitted from pip arg list

This code:
```
    if name.endswith('.tar.gz') or name.endswith('.tar.bz2') or
name.endswith('.zip'):
        is_tar = True
```
was not checking whether name is defined since it is an
optional param.
Merge pull request #2955 from jarv/devel

pip module blowing up when name is not included in arg list