Fix resolve-url for hash url

Doesn't work compiling with resolve-url flag for hash url:

```
$ cat ../hash.styl
// For IE8. Doesn't work hover on padding.
.my-class
	background-image url('#')

$ bin/stylus --resolve-url ../hash.styl

/home/rakchaev/projects/stylus/bin/stylus:646
              throw err;
                    ^
TypeError: ../hash.styl:3:26
   1| // For IE8. Doesn't work hover on padding.
   2| .my-class
   3| 	background-image url('#')
-------------------------------^
   4| 

Cannot call method 'substr' of null
    at ".my-class" (../hash.styl:2:1)
```