selftests/bpf: add missing pointer dereference for map stacktrace fixup

I get a segfault without it, other fixups always do dereference, and
without dereference I don't understand how it can ever work.

Fixes: 7c85c448 ("selftests/bpf: test_verifier, check bpf_map_lookup_elem access in bpf prog")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>