bpf, libbpf: fixing leak when kernel does not support btf

We could end up in situation when we have object file w/ all btf
info, but kernel does not support btf yet. In this situation
currently libbpf just set obj->btf to NULL w/o freeing it first.
This patch is fixing it by making sure to run btf__free first.

Fixes: d29d87f7e6 ("btf: separate btf creation and loading")
Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -838,6 +838,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 			if (IS_ERR(obj->btf) || btf__load(obj->btf)) {
 				pr_warning("Error loading ELF section %s: %ld. Ignored and continue.\n",
 					   BTF_ELF_SEC, PTR_ERR(obj->btf));
+				if (!IS_ERR(obj->btf))
+					btf__free(obj->btf);
 				obj->btf = NULL;
 			}
 		} else if (strcmp(name, BTF_EXT_ELF_SEC) == 0) {