lavc/ass_split: check for NULL pointer in ff_ass_split_override_codes().

This is consistent with the other ff_ass_split_* functions.

It also fixes a crash when trying to split a dialog with text=NULL
(which seems to happen when the text of the dialog is empty); basically,
this commit fixes crashes when trying to encode an empty text subtitle
dialog (see subrip and mov_text encoders).

Fixes Ticket2048.
