avcodec/hevc: Fix integer overflow of entry_point_offset

Fixes out of array read
Fixes: d41d8cd98f00b204e9800998ecf8427e/signal_sigsegv_321165b_7641_077dfcd8cbc80b1c0b470c8554cd6ffb.bit

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