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/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
