utils: verify next_url without encoding it

src/authentic2/utils/__init__.py

@@ -940,17 +940,21 @@ def good_next_url(request, next_url):
     return False
 
 
+def is_ascii(something):
+    try:
+        something.encode('ascii')
+        return True
+    except UnicodeEncodeError:
+        return False
+
+
 def get_next_url(params, field_name=None):
-    field_name = field_name or REDIRECT_FIELD_NAME
     '''Extract and decode a next_url field'''
+    field_name = field_name or REDIRECT_FIELD_NAME
     next_url = params.get(field_name)
     if not next_url:
         return None
-    try:
-        next_url = next_url.encode('ascii')
-    except UnicodeEncodeError:
-        return None
-    if not is_valid_url(next_url):
+    if not is_ascii(next_url) or not is_valid_url(next_url):
         return None
     return next_url