audio: fix wavcapture segfault

Commit 571a8c522e caused the HMP wavcapture command to segfault when
processing audio data in audio_pcm_sw_write(), where a NULL
sw->hw->pcm_ops is dereferenced. This fix checks that the pointer is
valid before dereferincing it. A similar fix is also made in the
parallel function audio_pcm_sw_read().

Fixes: 571a8c522e (audio: split ctl_* functions into enable_* and
volume_*)
Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200521172931.121903-1-brogers@suse.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/audio.c

@@ -649,7 +649,7 @@ static size_t audio_pcm_sw_read(SWVoiceIn *sw, void *buf, size_t size)
         total += isamp;
     }
 
-    if (!hw->pcm_ops->volume_in) {
+    if (hw->pcm_ops && !hw->pcm_ops->volume_in) {
         mixeng_volume (sw->buf, ret, &sw->vol);
     }
 
@@ -736,7 +736,7 @@ static size_t audio_pcm_sw_write(SWVoiceOut *sw, void *buf, size_t size)
     if (swlim) {
         sw->conv (sw->buf, buf, swlim);
 
-        if (!sw->hw->pcm_ops->volume_out) {
+        if (sw->hw->pcm_ops && !sw->hw->pcm_ops->volume_out) {
             mixeng_volume (sw->buf, swlim, &sw->vol);
         }
     }