Fix potential OOB vulnerability when SBC framedata is prefetched

Macros were filling a value buffer when reading bit stream data in
advance of any fetch being called, updated to only fetch data into
buffer when needed.

Bug: 110107252
Bug: 146032016
Bug: 146398979
Test: sbcdecoder_fuzzer
Change-Id: Ibb7793c9a015eaddcc8faa6ccfd8ec998d03ed86