libbpf: Fix error handling in btf__distill_base()

Coverity points out that after calling btf__new_empty_split() the wrong
value is checked for error.

Fixes: 58e185a0dc ("libbpf: Add btf__distill_base() creating split BTF with distilled base BTF")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240629100058.2866763-1-alan.maguire@oracle.com

tools/lib/bpf/btf.c

@@ -5431,7 +5431,7 @@ int btf__distill_base(const struct btf *src_btf, struct btf **new_base_btf,
 	 * BTF available.
 	 */
 	new_split = btf__new_empty_split(new_base);
-	if (!new_split_btf) {
+	if (!new_split) {
 		err = -errno;
 		goto done;
 	}