ossaudio: prevent SIGSEGV in oss_enable_out

With audiodev parameter out.mixing-engine=off hw->mix_buf is
NULL. This patch reverts a small part of dc88e38fa7 "audio:
unify input and output mixeng buffer management".

To reproduce the problem start qemu with
-audiodev oss,id=audio0,try-mmap=on,out.mixing-engine=off

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20200123074943.6699-6-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/ossaudio.c

@@ -592,7 +592,7 @@ static void oss_enable_out(HWVoiceOut *hw, bool enable)
             return;
         }
 
-        audio_pcm_info_clear_buf(&hw->info, hw->buf_emul, hw->mix_buf->size);
+        audio_pcm_info_clear_buf(&hw->info, hw->buf_emul, hw->samples);
         trig = PCM_ENABLE_OUTPUT;
         if (ioctl(oss->fd, SNDCTL_DSP_SETTRIGGER, &trig) < 0) {
             oss_logerr(errno,