Fail to parse .ass file when some margin is not an integer #443
Description
If a margin value in a .ass file is not an integer, eg. it is empty (",,") or a float (",25.00,") the parser fails with a "bad lexical cast" error.
Please find attached a minimal file to reproduce (had to change the ext to ".txt" for GitHub to accept it).
I already have a working fix. Pull request incoming.