libbpf: Fix NULL pointer exception in API btf_dump__dump_type_data

We found that function btf_dump__dump_type_data can be called by the
user as an API, but in this function, the `opts` parameter may be used
as a null pointer.This causes `opts->indent_str` to trigger a NULL
pointer exception.

Fixes: 2ce8450 ("libbpf: add bpf_object__open_{file, mem} w/ extensible opts")
Signed-off-by: Xin Liu <liuxin350@huawei.com>
Signed-off-by: Weibin Kong <kongweibin2@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220917084809.30770-1-liuxin350@huawei.com

tools/lib/bpf/btf_dump.c

@@ -2385,7 +2385,7 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
 	d->typed_dump->indent_lvl = OPTS_GET(opts, indent_level, 0);
 
 	/* default indent string is a tab */
-	if (!opts->indent_str)
+	if (!OPTS_GET(opts, indent_str, NULL))
 		d->typed_dump->indent_str[0] = '\t';
 	else
 		libbpf_strlcpy(d->typed_dump->indent_str, opts->indent_str,