libbpf: Fix potential NULL dereference when parsing ELF

Fix if condition filtering empty ELF sections to prevent NULL
dereference.

Fixes: 47ea7417 ("libbpf: Skip empty sections in bpf_object__init_global_data_maps")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20220816001929.369487-2-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -1649,7 +1649,7 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 		sec_desc = &obj->efile.secs[sec_idx];
 
 		/* Skip recognized sections with size 0. */
-		if (sec_desc->data && sec_desc->data->d_size == 0)
+		if (!sec_desc->data || sec_desc->data->d_size == 0)
 			continue;
 
 		switch (sec_desc->sec_type) {