avcodec/movtextdec: run mov_text_cleanup() before overwriting pointers

Fixes: memleak
Fixes: 741/clusterfuzz-testcase-586996200452915

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