matroskaenc: fix memory leak

This fixes a memory leak occurring when no cue points are defined
since commit 91819763.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/matroskaenc.c

@@ -403,8 +403,6 @@ static int64_t mkv_write_cues(AVIOContext *pb, mkv_cues *cues, int num_tracks)
     }
     end_ebml_master(pb, cues_element);
 
-    av_free(cues->entries);
-    av_free(cues);
     return currentpos;
 }
 
@@ -1160,6 +1158,8 @@ static int mkv_write_trailer(AVFormatContext *s)
 
     end_ebml_master(pb, mkv->segment);
     av_free(mkv->tracks);
+    av_freep(&mkv->cues->entries);
+    av_freep(&mkv->cues);
     av_destruct_packet(&mkv->cur_audio_pkt);
     avio_flush(pb);
     return 0;