avcodec/mlpdec: Check that there is enough data for headers

Fixes: out of array access
Fixes: 1541/clusterfuzz-testcase-minimized-6403410590957568

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