crash on startup on Debian 10 with FFmpeg enabled

Describe the bug

Following the default build instructions on Linux, I get a binary that crashes at start:

$ ./audacity
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
ALSA lib pcm_route.c:869:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:869:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:869:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:869:(find_matching_chmap) Found no matching channel map
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
ALSA lib pcm_oss.c:377:(_snd_pcm_oss_open) Unknown field port
ALSA lib pcm_oss.c:377:(_snd_pcm_oss_open) Unknown field port
ALSA lib pcm_a52.c:823:(_snd_pcm_a52_open) a52 is only for playback
Segmentation fault

(same when I have jack server running - just the jack messages disappear, but ends equally in Segmentation fault).

To Reproduce

1. clone repository, master branch, last commit: 8e16d87442538db3a665b834398c36ae3ff1e505
2. cd audacity
3. mkdir build && cd build
4. cmake -G "Unix Makefiles" -Daudacity_use_ffmpeg=loaded ..
5. ```make -j`nproc````
6. cd bin/Debug
7. mkdir "Portable Settings"
8. ./audacity

Additional information (please complete the following information):

• OS: Debian 10 (Linux holiday 4.19.0-16-amd64 HASH 1 SMP Debian 4.19.181-1 (2021-03-19) x86_64 GNU/Linux)
• CMake 3.20.5
• g++ (Debian 8.3.0-6) 8.3.0

In /var/log/syslog:

Jul  5 17:16:06 holiday org.gnome.Shell.desktop[1560]: Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x5200226 (Audacity i)
Jul  5 17:16:07 holiday kernel: [3958896.595781] audacity[8630]: segfault at 0 ip 0000000000000000 sp 00007ffd0f7ef798 error 14 in audacity[55adb09d6000+ca0000]
Jul  5 17:16:07 holiday kernel: [3958896.595785] Code: Bad RIP value.