libbpf: don't crash on object files with no symbol tables

If libbpf encounters an ELF file that has been stripped of its symbol
table, it will crash in bpf_object__add_programs() when trying to
dereference the obj->efile.symbols pointer. Add a check and return to avoid
this.

Fixes: 6245947 ("libbpf: Allow gaps in BPF program sections to support overriden weak functions")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

tools/lib/bpf/libbpf.c

@@ -668,6 +668,9 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
 	const char *name;
 	GElf_Sym sym;
 
+	if (!symbols)
+		return -ENOENT;
+
 	progs = obj->programs;
 	nr_progs = obj->nr_programs;
 	nr_syms = symbols->d_size / sizeof(GElf_Sym);