fix: MKV subtitle track .(null) extension for KATE and unknown codec IDs (#2250)

* fix: MKV subtitle track .(null) extension for KATE and unknown codec IDs

The matroska_track_text_subtitle_id_extensions array had 7 entries for
an 8-value enum, leaving MATROSKA_TRACK_SUBTITLE_CODEC_ID_KATE (index 7)
out of bounds. On most platforms this read NULL, which then caused
strlen(NULL) UB and snprintf to emit .(null) in the output filename.

Two fixes:
- Add "kate" at index 7 in the extensions array so KATE tracks
  produce correct .kate output filenames
- Add a NULL guard in generate_filename_from_track() so any future
  unknown codec ID safely falls back to .bin instead of crashing or
  producing .(null)

Fixes #972

* fix: MKV subtitle track .(null) extension for KATE and unknown codec IDs

The matroska_track_text_subtitle_id_extensions array had 7 entries for
an 8-value enum, leaving MATROSKA_TRACK_SUBTITLE_CODEC_ID_KATE (index 7)
out of bounds. On most platforms this read NULL, which then caused
strlen(NULL) UB and snprintf to emit .(null) in the output filename.

Two fixes:
- Add "kate" at index 7 in the extensions array so KATE tracks
  produce correct .kate output filenames
- Add a NULL guard in generate_filename_from_track() so any future
  unknown codec ID safely falls back to .bin instead of crashing or
  producing .(null)

Fixes #972

* fix: MKV subtitle track .(null) extension for KATE and unknown codec IDs

The matroska_track_text_subtitle_id_extensions array had 7 entries for
an 8-value enum, leaving MATROSKA_TRACK_SUBTITLE_CODEC_ID_KATE (index 7)
out of bounds. On most platforms this read NULL, which then caused
strlen(NULL) UB and snprintf to emit .(null) in the output filename.

Two fixes:
- Add "kate" at index 7 in the extensions array so KATE tracks
  produce correct .kate output filenames
- Add a NULL guard in generate_filename_from_track() so any future
  unknown codec ID safely falls back to .bin instead of crashing or
  producing .(null)

Fixes #972

---------

Co-authored-by: Dhanush Varma <your@email.com>

Author: DhanushVarma-2

src/lib_ccx/matroska.c

@@ -1333,6 +1333,8 @@ char *generate_filename_from_track(struct matroska_ctx *mkv_ctx, struct matroska
 {
 	const char *basename = get_basename(mkv_ctx->filename);
 	const char *extension = matroska_track_text_subtitle_id_extensions[track->codec_id];
+	if (extension == NULL)
+		extension = "bin";
 	/* Prefer the BCP-47 IETF tag (e.g. "zh-Hant") over the legacy
 	 * ISO-639-2 code (e.g. "chi") when one is available. */
 	const char *lang_tag = track->lang_ietf ? track->lang_ietf : track->lang;

src/lib_ccx/matroska.h

@@ -179,7 +179,7 @@ char *matroska_track_text_subtitle_id_strings[] = {
 char *matroska_track_text_subtitle_id_extensions[] = {
     "srt", "ssa", "ass",
     "usf", "vtt", "bmp",
-    "idx", NULL // Unknown
+    "idx", "kate", NULL // Unknown/sentinel
 };
 
 char *avc_codec_id = "V_MPEG4/ISO/AVC";