fix: Add syntax check for SCTE 20 marker bit

This adds a validation check for the SCTE 20 marker bit in es_userdata.c. If the bit is 0, a debug warning is logged. This prevents potential parsing issues with corrupt streams.

Author: AhmedAlian7

docs/CHANGES.TXT

@@ -12,6 +12,7 @@
 - Fix: Delete empty output files instead of leaving 0-byte files (#1282)
 - Fix: --mkvlang now supports BCP 47 language tags (e.g., en-US, zh-Hans-CN) and multiple codes
 - Fix: segmentation fault when using --multiprogram
+- Fix: Add syntax check for SCTE 20 marker bit to prevent incorrect parsing of corrupt streams
 
 0.96.5 (2026-01-05)
 -------------------

src/lib_ccx/es_userdata.c

@@ -161,7 +161,10 @@ int user_data(struct encoder_ctx *enc_ctx, struct lib_cc_decode *dec_ctx, struct
 				skip_bits(ustream, 5); // line_offset - unused
 				cc_data1 = (unsigned int)read_bits(ustream, 8);
 				cc_data2 = (unsigned int)read_bits(ustream, 8);
-				read_bits(ustream, 1); // TODO: Add syntax check */
+				if (read_bits(ustream, 1) == 0)
+				{
+					dbg_print(CCX_DMT_VERBOSE, "SCTE 20: Marker bit is 0 (should be 1)\n");
+				}
 
 				if (ustream->bitsleft < 0)
 					fatal(CCX_COMMON_EXIT_BUG_BUG, "In user_data: ustream->bitsleft < 0. Cannot continue.");