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: 13edc237 ("perf bench: Add a multi-threaded synthesize benchmark")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200902140526.26916-1-yuehaibing@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/bench/synthesize.c

@@ -162,8 +162,8 @@ static int do_run_multi_threaded(struct target *target,
 	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);