avcodec/movtextdec: Add error message for tsmb_size check

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 0eb3198005)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/movtextdec.c

@@ -487,6 +487,7 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
             //size_var is equal to 8 or 16 depending on the size of box
 
             if (tsmb_size == 0) {
+                av_log(avctx, AV_LOG_ERROR, "tsmb_size is 0\n");
                 return AVERROR_INVALIDDATA;
             }