tools: bpftool: fix crash on bad parameters with JSON

If bad or unrecognised parameters are specified after JSON output is
requested, `usage()` will try to output null JSON object before the
writer is created.

To prevent this, create the writer as soon as the `--json` option is
parsed.

Fixes: 004b45c0 ("tools: bpftool: provide JSON output for all possible commands")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/main.c

@@ -291,7 +291,15 @@ int main(int argc, char **argv)
 			pretty_output = true;
 			/* fall through */
 		case 'j':
+			if (!json_output) {
+				json_wtr = jsonw_new(stdout);
+				if (!json_wtr) {
+					p_err("failed to create JSON writer");
+					return -1;
+				}
 				json_output = true;
+			}
+			jsonw_pretty(json_wtr, pretty_output);
 			break;
 		case 'f':
 			show_pinned = true;
@@ -306,15 +314,6 @@ int main(int argc, char **argv)
 	if (argc < 0)
 		usage();
 
-	if (json_output) {
-		json_wtr = jsonw_new(stdout);
-		if (!json_wtr) {
-			p_err("failed to create JSON writer");
-			return -1;
-		}
-		jsonw_pretty(json_wtr, pretty_output);
-	}
-
 	bfd_init();
 
 	ret = cmd_select(cmds, argc, argv, do_help);