[FIX] website_slide: fix traceback when slide is a google doc or youtube link

In case we are not uploading a file but add a slide based on google doc url
or youtube link, post['datas'] is not set and the code will crash.

Javascript don't check the onError, so the user never see the error and wait
 that nothing happend.

addons/website_slides/controllers/main.py

@@ -292,9 +292,11 @@ class website_slides(http.Controller):
 
     @http.route(['/slides/add_slide'], type='json', auth='user', methods=['POST'], website=True)
     def create_slide(self, *args, **post):
-        file_size = len(post['datas']) * 3 / 4; # base64
-        if (file_size / 1024.0 / 1024.0) > 25:
-            return {'error': _('File is too big. File size cannot exceed 25MB')}
+        # check the size only when we upload a file.
+        if post.get('datas'):
+            file_size = len(post['datas']) * 3 / 4 # base64
+            if (file_size / 1024.0 / 1024.0) > 25:
+                return {'error': _('File is too big. File size cannot exceed 25MB')}
 
         values = dict((fname, post[fname]) for fname in [
             'name', 'url', 'tag_ids', 'slide_type', 'channel_id',