perf bench: The do_run_multi_threaded() function must use IS_ERR(perf_session__new())

In case of error, the function perf_session__new() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR()

Committer notes:

This wasn't compiling due to an extraneous '{' not matched by a '}', fix
it.

Fixes: 13edc237200c ("perf bench: Add a multi-threaded synthesize benchmark")
Signed-off-by: YueHaibing <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/bench/synthesize.c

···
 	init_stats(&event_stats);
 	for (i = 0; i < multi_iterations; i++) {
 		session = perf_session__new(NULL, false, NULL);
-		if (!session)
-			return -ENOMEM;
+		if (IS_ERR(session))
+			return PTR_ERR(session);
 
 		atomic_set(&event_count, 0);
 		gettimeofday(&start, NULL);