libbpf: Fix off-by-one bug in bpf_core_apply_relo()

Fix instruction index validity check which has off-by-one error.

Fixes: 3ee4f533

 ("libbpf: Split bpf_core_apply_relo() into bpf_program independent helper.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211025224531.1088894-2-andrii@kernel.org