libbpf: Skip empty sections in bpf_object__init_global_data_maps

The GNU assembler generates an empty .bss section. This is a well
established behavior in GAS that happens in all supported targets.

The LLVM assembler doesn't generate an empty .bss section.

bpftool chokes on the empty .bss section.

Additionally in bpf_object__elf_collect the sec_desc->data is not
initialized when a section is not recognized. In this case, this
happens with .comment.

So we must check that sec_desc->data is initialized before checking
if the size is 0.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20220731232649.4668-1-james.hilliard1@gmail.com

tools/lib/bpf/libbpf.c

@@ -1642,6 +1642,10 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 	for (sec_idx = 1; sec_idx < obj->efile.sec_cnt; sec_idx++) {
 		sec_desc = &obj->efile.secs[sec_idx];
 
+		/* Skip recognized sections with size 0. */
+		if (sec_desc->data && sec_desc->data->d_size == 0)
+			continue;
+
 		switch (sec_desc->sec_type) {
 		case SEC_DATA:
 			sec_name = elf_sec_name(obj, elf_sec_by_idx(obj, sec_idx));