libbpf: Prevent null-pointer dereference when prog to load has no BTF

In the Linux kernel, the following vulnerability has been resolved:

libbpf: Prevent null-pointer dereference when prog to load has no BTF

In bpf_objec_load_prog(), there's no guarantee that obj->btf is non-NULL when passing it to btf__fd(), and this function does not perform any check before dereferencing its argument (as bpf_object__btf_fd() used to do). As a consequence, we get segmentation fault errors in bpftool (for example) when trying to load programs that come without BTF information.

v2: Keep btf__fd() in the fix instead of reverting to bpf_object__btf_fd().