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.

libavcodec/ass_split.c

@@ -369,7 +369,7 @@ int ff_ass_split_override_codes(const ASSCodesCallbacks *callbacks, void *priv,
     char new_line[2];
     int text_len = 0;
 
-    while (*buf) {
+    while (buf && *buf) {
         if (text && callbacks->text &&
             (sscanf(buf, "\\%1[nN]", new_line) == 1 ||
              !strncmp(buf, "{\\", 2))) {