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().

Fixes: df7c3f7d3a3d ("libbpf: make uniform use of btf__fd() accessor inside libbpf")
Suggested-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Quentin Monnet <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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