selftests/bpf: Check overflow in optional buffer

This ensures we still reject invalid memory accesses in buffers that are
marked optional.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20230506013134.2492210-4-drosen@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>