movenc: Fix unfreed memory found by valgrind.

Signed-off-by: Philip Langdale <philipl@overt.org>

libavformat/movenc.c

@@ -3154,11 +3154,12 @@ static int mov_write_subtitle_end_packet(AVFormatContext *s,
                                          int stream_index,
                                          int64_t dts) {
     AVPacket end;
+    short data = 0;
     int ret;
 
     av_init_packet(&end);
     end.size = sizeof (short);
-    end.data = av_mallocz(end.size);
+    end.data = (char *)&data;
     end.pts = dts;
     end.dts = dts;
     end.duration = 0;