perf tools: Remove the '--(null)' long_name for --list-opts

If the long_name of a 'struct option' is defined as NULL, --list-opts
will incorrectly print '--(null)' in its output. As a result, '--(null)'
will finally appear in the case of bash completion, e.g. 'perf record
--'.

Example:

Before this patch:

 $ perf record --list-opts

 --event --filter --pid --tid --realtime --no-buffering --raw-samples
 --all-cpus --cpu --count --output --no-inherit --freq --mmap-pages
 --group --(null) --call-graph --verbose --quiet --stat --data
 --timestamp --period --no-samples --no-buildid-cache --no-buildid
 --cgroup --delay --uid --branch-any --branch-filter --weight
 --transaction --per-thread --intr-regs

After this patch:

 $ perf record --list-opts

 --event --filter --pid --tid --realtime --no-buffering --raw-samples
 --all-cpus --cpu --count --output --no-inherit --freq --mmap-pages
 --group --call-graph --verbose --quiet --stat --data --timestamp
 --period --no-samples --no-buildid-cache --no-buildid --cgroup --delay
 --uid --branch-any --branch-filter --weight --transaction --per-thread
 --intr-regs

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1425032491-20224-7-git-send-email-yunlong.song@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-options.c

@@ -505,7 +505,8 @@ int parse_options_subcommand(int argc, const char **argv, const struct option *o
 		break;
 	case PARSE_OPT_LIST_OPTS:
 		while (options->type != OPTION_END) {
-			printf("--%s ", options->long_name);
+			if (options->long_name)
+				printf("--%s ", options->long_name);
 			options++;
 		}
 		putchar('\n');