avcodec/hevc: clear HEVClc when its deallocated in hevc_decode_free()

Fixes reading freed memory
Fixes: asan_heap-uaf_1abf8ef_3987_NUT_A_ericsson_4.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
hevc: store profile and level in AVCodecContext

Signed-off-by: Anton Khirnov <anton@khirnov.net>

Conflicts:
	libavcodec/hevc.c