selftests: bpf: initialize bpf_object pointers where needed

There are a few tests which call bpf_object__close on uninitialized
bpf_object*, which may segfault. Explicitly zero-initialise these pointers
to avoid this.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>