Validate lengths in HEVC metadata parsing

Add code to validate the size parameter passed to
HecvParameterSets::addNalUnit().  Previously vulnerable
to decrementing an unsigned past 0, yielding a huge result value.

Bug: 35467107
Test: ran POC, no crash, emitted new "bad length" log entry
Change-Id: Ia169b9edc1e0f7c5302e3c68aa90a54e8863d79e