libbpf: fix SIGSEGV when BTF loading fails, but .BTF.ext exists
In case when BTF loading fails despite sanitization, but BPF object has
.BTF.ext loaded as well, we free and null obj->btf, but not
obj->btf_ext. This leads to an attempt to relocate .BTF.ext later on
during bpf_object__load(), which assumes obj->btf is present. This leads
to SIGSEGV on null pointer access. Fix bug by freeing and nulling
obj->btf_ext as well.