Panic when workspace path contains reserved characters

### Version
exercism-3.0.13-windows-x86_64

### Os
Windows 7

### Reproduce
`exercism.exe configure --workspace "c:\?"`

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x18 pc=0x84d9e9]

goroutine 1 [running]:
github.com/exercism/cli/cmd.runConfigure(0x95ba0b, 0x7, 0xc00006e4a0, 0xf, 0xc0000861e0, 0x28, 0x967b7a, 0x1a, 0xc0000706d0, 0x8, ...)
/Users/ekingery/dev/exercism/cli/cmd/configure.go:147 +0x959
github.com/exercism/cli/cmd.glob..func1(0xdd5cc0, 0xc000064e40, 0x0, 0x2, 0x0, 0x0)
/Users/ekingery/dev/exercism/cli/cmd/configure.go:43 +0x167
github.com/spf13/cobra.(*Command).execute(0xdd5cc0, 0xc000064e00, 0x2, 0x2, 0xdd5cc0, 0xc000064e00)
/Users/ekingery/golang/pkg/mod/github.com/spf13/cobra@v0.0.0-20170731170427-b26b538f6930/command.go:649 +0x40f
github.com/spf13/cobra.(*Command).ExecuteC(0xdd65c0, 0x0, 0x0, 0x0)
/Users/ekingery/golang/pkg/mod/github.com/spf13/cobra@v0.0.0-20170731170427-b26b538f6930/command.go:728 +0x2c3
github.com/spf13/cobra.(*Command).Execute(...)
/Users/ekingery/golang/pkg/mod/github.com/spf13/cobra@v0.0.0-20170731170427-b26b538f6930/command.go:687
github.com/exercism/cli/cmd.Execute()
/Users/ekingery/dev/exercism/cli/cmd/root.go:39 +0x34
main.main()
/Users/ekingery/dev/exercism/cli/exercism/main.go:6 +0x27
```