Avoid a null pointer dereference on clean-up after oom in ac3 encoder.

libavcodec/ac3enc.c

@@ -2017,6 +2017,7 @@ av_cold int ff_ac3_encode_close(AVCodecContext *avctx)
     AC3EncodeContext *s = avctx->priv_data;
 
     av_freep(&s->windowed_samples);
+    if (s->planar_samples)
     for (ch = 0; ch < s->channels; ch++)
         av_freep(&s->planar_samples[ch]);
     av_freep(&s->planar_samples);