libbpf: fix signedness determination in CO-RE relo handling logic

Extracting btf_int_encoding() is only meaningful for BTF_KIND_INT, so we
need to check that first before inferring signedness.

Closes: https://github.com/libbpf/libbpf/issues/704
Reported-by: Lorenz Bauer <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/lib/bpf/relo_core.c

···
 		break;
 	case BPF_CORE_FIELD_SIGNED:
 		*val = (btf_is_any_enum(mt) && BTF_INFO_KFLAG(mt->info)) ||
-		       (btf_int_encoding(mt) & BTF_INT_SIGNED);
+		       (btf_is_int(mt) && (btf_int_encoding(mt) & BTF_INT_SIGNED));
 		if (validate)
 			*validate = true; /* signedness is never ambiguous */
 		break;