avcodec/mlpdec: Check quant_step_size against huff_lsbs

This reorders the operations so as to avoid computations with the above arguments
before they have been initialized.
Fixes part of 1708/clusterfuzz-testcase-minimized-5035111957397504

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