alsamixer: segfault in display_controls()

alsamixer: segfault in display_controls()

[GitHub issues - opened]

alsa-project/alsa-utils issue #335 was opened from fenugrec:

I think I know what sequence triggers this.

*** steps to reproduce
1-  connect a device that has no Capture controls (unclear if this is the crucial difference). Here this is a Presonus Studio 1824c
2- run alsamixer; F6 to select that Card
3- Go the Capture view (F4)
4- arrow right
5- go to All view (F5) -> segfault


*** backtrace
```
#0  display_control (control_index=0xffffffff) at /usr/src/debug/alsa-utils/alsa-utils-1.2.16/alsamixer/mixer_display.c:421
        control = 0x55855558ff60
        col = 0x0
        i = <optimized out>
        c = <optimized out>
        left = 0x2
        frame_left = 0xb
        bar_height = <optimized out>
        volumes = {0, 6.9531436082559572e-310}
        switches = {0x0, 0x0}
        index = 0x555a0710
        s = <optimized out>
        buf = "\360\030_UUU\000\000\003\000\000\000UU", '\000' <repeats 11 times>, "\230z\021\003\000\000\000\b\000\000\000\a\000\000\000\003\000\000\000UU\000\000\2352VUUU\000\000\2462VUUU\000"
        err = <optimized out>
#1  display_controls () at /usr/src/debug/alsa-utils/alsa-utils-1.2.16/alsamixer/mixer_display.c:674
        i = 0x0
#2  0x00005555555585dc in mainloop () at /usr/src/debug/alsa-utils/alsa-utils-1.2.16/alsamixer/mainloop.c:146
        pollfds = 0x55555560e790
        nfds = 0x2
        n = <optimized out>
        active_widget = 0x5555555676e0 <mixer_widget>
        revents = 0x1
        key = <optimized out>
        err = <optimized out>
#3  main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/alsa-utils/alsa-utils-1.2.16/alsamixer/cli.c:176
No locals.
```

The problem is the argument `control_index=0xffffffff` which comes from display_controls() around lines 660-670 ; in this debug run (after the segfault) I observe the following values

```
(gdb) p first_visible_control_index
$1 = 0xffffffff
(gdb) p controls_count
$2 = 0x5
(gdb) p visible_controls
$3 = 0x5
(gdb) p focus_control_index
$4 = 0x0
```

not sure why `first_visible_control_index` has that value...

Issue URL     : https://github.com/alsa-project/alsa-utils/issues/335
Repository URL: https://github.com/alsa-project/alsa-utils