selftests/bpf: Fix possible NULL passed to memcpy() with zero size

Prevent sanitizer from complaining about passing NULL into memcpy(),
even if it happens with zero size.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-9-andrii@kernel.org