libbpf: Verify that arena map exists when adding arena relocations

Fuzzer reported a memory access error in bpf_program__record_reloc()
that happens when:
- ".addr_space.1" section exists
- there is a relocation referencing this section
- there are no arena maps defined in BTF.

Sanity checks for maps existence are already present in
bpf_program__record_reloc(), hence this commit adds another one.

[1] https://github.com/libbpf/libbpf/actions/runs/16375110681/job/46272998064

Signed-off-by: Eduard Zingerman <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: eddyz87

tools/lib/bpf/libbpf.c

@@ -4582,6 +4582,11 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 
 	/* arena data relocation */
 	if (shdr_idx == obj->efile.arena_data_shndx) {
+		if (obj->arena_map_idx < 0) {
+			pr_warn("prog '%s': bad arena data relocation at insn %u, no arena maps defined\n",
+				prog->name, insn_idx);
+			return -LIBBPF_ERRNO__RELOC;
+		}
 		reloc_desc->type = RELO_DATA;
 		reloc_desc->insn_idx = insn_idx;
 		reloc_desc->map_idx = obj->arena_map_idx;