audio: Do not segfault in out_get_presentation_position()

When a voice call is stopped we switch back to the default primary
output (speaker). Then this function gets executed and as ther was
no active PCM because voice_session is handling that it segfaults
because the PCM is NULL.

Change-Id: I927504b7962b096c0d4c3642b48aee55c85ec013

audio/audio_hw.c

@@ -3086,10 +3086,16 @@ static int out_get_presentation_position(const struct audio_stream_out *stream,
     } else {
         /* FIXME: which device to read from? */
         if (!list_empty(&out->pcm_dev_list)) {
+            struct pcm_device *pcm_device;
+            struct listnode *node;
             unsigned int avail;
-            struct pcm_device *pcm_device = node_to_item(list_head(&out->pcm_dev_list),
-                                                   struct pcm_device, stream_list_node);
 
+            list_for_each(node, &out->pcm_dev_list) {
+                pcm_device = node_to_item(node,
+                                          struct pcm_device,
+                                          stream_list_node);
+
+                if (pcm_device->pcm != NULL) {
                     if (pcm_get_htimestamp(pcm_device->pcm, &avail, timestamp) == 0) {
                         size_t kernel_buffer_size = out->config.period_size * out->config.period_count;
                         int64_t signed_frames = out->written - kernel_buffer_size + avail;
@@ -3102,11 +3108,16 @@ static int out_get_presentation_position(const struct audio_stream_out *stream,
                         if (signed_frames >= 0) {
                             *frames = signed_frames;
                             ret = 0;
+                            goto done;
+                        }
+                        ret = -1;
+                    }
                 }
             }
         }
     }
 
+done:
     pthread_mutex_unlock(&out->lock);
 
     return ret;