alsactl: don't free a card pointing NULL

[masatake]

alsactl distributed as part of Fedora 40 got a SEGV:

# journalctl
...
May 17 00:55:58 dev64.localdomain kernel: alsactl[1923]: segfault at 28 ip 00005600705b3373 sp 00007ffd9712bef0 error 4 in alsactl[5600705af000+13000] likely on CPU 5 (core 8, socket 0)
...

As the following output of the debug session, card_free() tried a card pointing NULL:

$ sudo coredumpctl debug alsactl
       PID: 1923 (alsactl)
       UID: 0 (root)
       GID: 0 (root)
    Signal: 11 (SEGV)
 Timestamp: Fri 2024-05-17 00:55:58 JST (3h 34min ago)
  Command Line: /usr/sbin/alsactl -s -n 19 -c -E ALSA_CONFIG_PATH=/etc/alsa/alsactl.conf --initfile=/lib/alsa/init/00main rdaemon
Executable: /usr/sbin/alsactl
 Control Group: /system.slice/alsa-state.service
      Unit: alsa-state.service
     Slice: system.slice
   Boot ID: 241b5a2ef86f4940bb3d340583c80d88
Machine ID: 437365709a8c488c9481ee4b6651c2ec
  Hostname: dev64.localdomain
   Storage: /var/lib/systemd/coredump/core.alsactl.0.241b5a2ef86f4940bb3d340583c80d88.1923.1715874958000000.zst (present)
  Size on Disk: 81.7K
   Package: alsa-utils/1.2.11-1.fc40
  build-id: 3b6fec58b3566d666d6e9fd48e8fcf04f03f0152
   Message: Process 1923 (alsactl) of user 0 dumped core.

	    Module libasound.so.2 from rpm alsa-lib-1.2.11-2.fc40.x86_64
	    Module alsactl from rpm alsa-utils-1.2.11-1.fc40.x86_64
	    Stack trace of thread 1923:
	    #0  0x00005600705b3373 card_free (alsactl + 0xa373)
	    #1  0x00005600705c0e54 state_daemon (alsactl + 0x17e54)
	    #2  0x00005600705b2339 main (alsactl + 0x9339)
	    #3  0x00007f4c0b9b7088 __libc_start_call_main (libc.so.6 + 0x2a088)
	    #4  0x00007f4c0b9b714b __libc_start_main_impl (libc.so.6 + 0x2a14b)
	    #5  0x00005600705b2df5 _start (alsactl + 0x9df5)
	    ELF object binary architecture: AMD x86-64

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Reading symbols from /usr/sbin/alsactl...
Reading symbols from /usr/lib/debug/usr/sbin/alsactl-1.2.11-1.fc40.x86_64.debug...
[New LWP 1923]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Core was generated by `/usr/sbin/alsactl -s -n 19 -c -E ALSA_CONFIG_PATH=/etc/alsa/alsactl.conf --init'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  free_list (list=0x20) at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/daemon.c:73
73		for (i = 0; i < list->size; i++)
(gdb) where
#0  free_list (list=0x20) at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/daemon.c:73
#1  card_free (card=card@entry=0x5600707455f0) at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/daemon.c:82
#2  0x00005600705c0e54 in state_daemon (file=file@entry=0x5600705c31a1 "/var/lib/alsa/asound.state", cardname=cardname@entry=0x0, period=period@entry=300,
pidfile=pidfile@entry=0x5600705c3170 "/var/run/alsactl.pid") at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/daemon.c:455
#3  0x00005600705b2339 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/alsactl.c:459
(gdb) list
68
69	static void free_list(struct id_list *list)
70	{
71		int i;
72
73		for (i = 0; i < list->size; i++)
74			free(list->list[i]);
75		free(list->list);
76	}
77
(gdb) up
#1  card_free (card=card@entry=0x5600707455f0) at /usr/src/debug/alsa-utils-1.2.11-1.fc40.x86_64/alsactl/daemon.c:82
82		free_list(&c->blacklist);
(gdb) p c
$1 = (struct card *) 0x0
(gdb)