avcodec/hevc: Check entry_point_offsets

Fixes out of array read
Fixes: 007c4a36608ebdf27ee260ad60a81184/asan_heap-oob_32076b4_2243_116b1cb29d91cc4974d6680e3d10bd91.bit

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