fix: initialize demuxer_opts to NULL in transcode_init

[livepeer-tessa]

Problem

The demuxer_opts pointer in transcode_init() was left uninitialized when inp->demuxer.opts was NULL. This caused avformat_open_input to receive a garbage pointer, leading to a crash in av_dict_copy when processing dictionary options.

Root Cause

AVDictionary **demuxer_opts;  // UNINITIALIZED
if (inp->demuxer.opts) demuxer_opts = &inp->demuxer.opts;
// If inp->demuxer.opts is NULL, demuxer_opts is garbage
ret = avformat_open_input(&ictx->ic, inp->fname, fmt, demuxer_opts);  // CRASH

Fix

Initialize the pointer to NULL:

AVDictionary **demuxer_opts = NULL;

Testing

• TestAPI_ConsecutiveMP4s now passes (was crashing with SIGSEGV)
• This fixes the CI failure that has been occurring since at least December 2025

How it was found

Used GDB and AddressSanitizer to trace the crash:

1. ASan showed crash in avformat_open_input
2. GDB revealed actual crash location was av_dict_copy in libavutil
3. Traced back to uninitialized demuxer_opts pointer