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: libbpf/libbpf#704
Reported-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230824000016.2658017-2-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Author: anakryiko

tools/lib/bpf/relo_core.c

@@ -776,7 +776,7 @@ static int bpf_core_calc_field_relo(const char *prog_name,
 		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;