vcodec/h2645_parse: Clear buffer padding

Fixes use of uninitialized memory
Fixes: 044100cb22845944988a4bd821ff8074/asan_heap-oob_329927a_1366_c3de34ce9217dac820fbb46171031bbb.jsv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
