avcodec/movtextdec: Call mov_text_cleanup() on close

Fixes memleak
Fixes: 548/clusterfuzz-testcase-5511470875934720

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/movtextdec.c

@@ -521,6 +521,7 @@ static int mov_text_decode_close(AVCodecContext *avctx)
 {
     MovTextContext *m = avctx->priv_data;
     mov_text_cleanup_ftab(m);
+    mov_text_cleanup(m);
     return 0;
 }