libbpf: Fix the possible memory leak on error

If the strdup() fails then we need to call bpf_object__close(obj) to
avoid a resource leak.

Fixes: 166750bc

 ("libbpf: Support libbpf-provided extern variables")
Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626180159-112996-3-git-send-email-chengshuyi@linux.alibaba.com