movenc: fixes a questionable valgrind uninitialized value warning

display_matrix_size is only initialized when av_stream_get_side_data()
returns a side data pointer. The code is safe since the only effect this
has is setting the display_matrix pointer to NULL which it was already
anyway.

libavformat/movenc.c

@@ -1518,7 +1518,7 @@ static int mov_write_tkhd_tag(AVIOContext *pb, MOVMuxContext *mov,
 
         display_matrix = (uint32_t*)av_stream_get_side_data(st, AV_PKT_DATA_DISPLAYMATRIX,
                                                             &display_matrix_size);
-        if (display_matrix_size < 9 * sizeof(*display_matrix))
+        if (display_matrix && display_matrix_size < 9 * sizeof(*display_matrix))
             display_matrix = NULL;
     }