Fix and clean up opus_decode_fuzzer

Use the fuzzed sub-length of the input data instead of the whole input.
3 jobs for master in 4 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10414
docker
autoconf

00:04:47

passed #10415
docker
cmake

00:02:09

 
  External
passed continuous-integration/appveyor/branch