selftests/bpf: Fix btf leak on new btf alloc failure in btf_distill test

Fix btf leak on new btf alloc failure in btf_distill test.

Fixes: affdeb50 ("selftests/bpf: Extend distilled BTF tests to cover BTF relocation")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250115100241.4171581-1-pulehui@huaweicloud.com