Avoid breakage if Pdata is shorter than wanted size

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35047 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5SyncML/Signature.py

@@ -390,7 +390,7 @@ class Signature(Folder, SyncCode, File):
     chunk.append(self.partial_xml.data)
     size = chunk[0].count('\n')
     current = next = self.partial_xml.next
-    while size < size_lines:
+    while size < size_lines and next is not None:
       current = next
       size += current.data.count('\n')
       chunk.append(current.data)