[#3656] Fix broken default logo image

If you go to the sysadmin config page, and if you don't change the logo image and just click Update, the logo image will break.
The reason is because its been given wrong path to the image.

ckan/logic/action/update.py

@@ -7,6 +7,7 @@
 import time
 import json
 import mimetypes
+import os
 
 from ckan.common import config
 import paste.deploy.converters as converters
@@ -1274,11 +1275,25 @@ def config_option_update(context, data_dict):
         model.Session.rollback()
         raise ValidationError(errors)
 
+    image_in_ckan = False
+    ckan_images_path = os.path.abspath(os.path.join(os.path.dirname(__file__),
+                                       '..', '..', 'public', 'base', 'images'))
+
     for key, value in data.iteritems():
 
         # Set full Logo url
-        if key =='ckan.site_logo' and value and not value.startswith('http'):
-            value = h.url_for_static('uploads/admin/{0}'.format(value))
+        if key == 'ckan.site_logo' and value and not value.startswith('http'):
+            for f in os.listdir(ckan_images_path):
+                if f == value:
+                    image_in_ckan = True
+                    break
+
+            if image_in_ckan:
+                image_path = 'base/images/'
+            else:
+                image_path = 'uploads/admin/'
+
+            value = h.url_for_static('{0}{1}'.format(image_path, value))
 
         # Save value in database
         model.set_system_info(key, value)