avcodec/movtextdec: Fix decode_styl() cleanup

Fixes: null pointer dereference
Fixes: 555/clusterfuzz-testcase-5986646595993600

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