lavc: Fix use of uninitialized field.

Fixes CID732297
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -1753,10 +1753,14 @@ int attribute_align_arg avcodec_decode_audio4(AVCodecContext *avctx,
     /* many decoders assign whole AVFrames, thus overwriting extended_data;
      * make sure it's set correctly; assume decoders that actually use
      * extended_data are doing it correctly */
-    planar   = av_sample_fmt_is_planar(frame->format);
-    channels = av_get_channel_layout_nb_channels(frame->channel_layout);
-    if (!(planar && channels > AV_NUM_DATA_POINTERS))
-        frame->extended_data = frame->data;
+    if (*got_frame_ptr) {
+        planar   = av_sample_fmt_is_planar(frame->format);
+        channels = av_get_channel_layout_nb_channels(frame->channel_layout);
+        if (!(planar && channels > AV_NUM_DATA_POINTERS))
+            frame->extended_data = frame->data;
+    } else {
+        frame->extended_data = NULL;
+    }
 
     return ret;
 }