avcodec/movtextdec: Fix tsmb_size check==0 check

Fixes: 173/fuzz-3-ffmpeg_SUBTITLE_AV_CODEC_ID_MOV_TEXT_fuzzer

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a609905723)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/movtextdec.c

@@ -471,10 +471,6 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
             tsmb_type = AV_RB32(tsmb);
             tsmb += 4;
 
-            if (tsmb_size == 0) {
-              return AVERROR_INVALIDDATA;
-            }
-
             if (tsmb_size == 1) {
                 if (m->tracksize + 16 > avpkt->size)
                     break;
@@ -485,6 +481,10 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
                 m->size_var = 8;
             //size_var is equal to 8 or 16 depending on the size of box
 
+            if (tsmb_size == 0) {
+                return AVERROR_INVALIDDATA;
+            }
+
             if (tsmb_size > avpkt->size - m->tracksize)
                 break;