perf data convert: Fix segfault when converting to json when cpu_desc isn't set

Arm64 doesn't have Model in /proc/cpuinfo and, thus, cpu_desc doesn't get
assigned.

Running
$ perf data convert --to-json perf.data.json

ends up calling output_json_string() with NULL pointer, which causes a
segmentation fault.

Signed-off-by: Ilkka Koskinen <[e-mail]>
Acked-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: Evgeny Pistun <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/perf/util/data-convert-json.c

···
 	output_json_key_string(out, true, 2, "os-release", header->env.os_release);
 	output_json_key_string(out, true, 2, "arch", header->env.arch);
 
-	output_json_key_string(out, true, 2, "cpu-desc", header->env.cpu_desc);
+	if (header->env.cpu_desc)
+		output_json_key_string(out, true, 2, "cpu-desc", header->env.cpu_desc);
+
 	output_json_key_string(out, true, 2, "cpuid", header->env.cpuid);
 	output_json_key_format(out, true, 2, "nrcpus-online", "%u", header->env.nr_cpus_online);
 	output_json_key_format(out, true, 2, "nrcpus-avail", "%u", header->env.nr_cpus_avail);