libperf cpumap: Fix perf_cpu_map__max for an empty/NULL map

Passing an empty map to perf_cpu_map__max triggered a SEGV. Explicitly
test for the empty map.

Reported-by: Ingo Molnar <[e-mail]>
Closes: https://lore.kernel.org/linux-perf-users/[e-mail]/
Tested-by: Ingo Molnar <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Tested-by: Thomas Richter <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>

tools/lib/perf/cpumap.c

···
 		.cpu = -1
 	};
 
-	// cpu_map__trim_new() qsort()s it, cpu_map__default_new() sorts it as well.
-	return __perf_cpu_map__nr(map) > 0
-		? __perf_cpu_map__cpu(map, __perf_cpu_map__nr(map) - 1)
-		: result;
+	if (!map)
+		return result;
+
+	// The CPUs are always sorted and nr is always > 0 as 0 length map is
+	// encoded as NULL.
+	return __perf_cpu_map__cpu(map, __perf_cpu_map__nr(map) - 1);
 }
 
 /** Is 'b' a subset of 'a'. */