libbpf: Fix return zero when elf_begin failed

The error number of elf_begin is omitted when encapsulating the
btf_find_elf_sections function.

Fixes: c86f180f ("libbpf: Make btf_parse_elf process .BTF.base transparently")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250115100241.4171581-2-pulehui@huaweicloud.com