/api/delete_sr_image: Ensure valid name submitted.

Previously, if an invalid image name were passed to this endpoint, it
would fail because VCssName would return an empty string and we'd never
check for the validity of the value.

Fixes reddit/reddit#883.

r2/r2/controllers/api.py

@@ -1655,6 +1655,10 @@ class ApiController(RedditController, OAuth2ResourceController):
         # just in case we need to kill this feature from XSS
         if g.css_killswitch:
             return self.abort(403,'forbidden')
+
+        if form.has_errors("img_name", errors.BAD_CSS_NAME):
+            return
+
         wiki.ImagesByWikiPage.delete_image(c.site, "config/stylesheet", name)
         ModAction.create(c.site, c.user, action='editsettings', 
                          details='del_image', description=name)