demux/subtitle: Fix wrong ASS header parsing

After optimization in 6cccd64d,
complete ASS header was discarded for some subtitles.

close #12733

modules/demux/subtitle.c

@@ -1156,6 +1156,9 @@ static int  ParseSSA( demux_t *p_demux, subtitle_t *p_subtitle,
         free( psz_text );
 
         /* All the other stuff we add to the header field */
+        if( header_len == 0 && p_sys->psz_header )
+            header_len = strlen( p_sys->psz_header );
+
         size_t s_len = strlen( s );
         p_sys->psz_header = realloc_or_free( p_sys->psz_header, header_len + s_len + 2 );
         if( !p_sys->psz_header )