libperf threadmap: Don't segv for index 0 for the NULL 'struct perf_thread_map' pointer

perf_thread_map__nr() returns length 1 if the perf_thread_map is NULL,
meaning index 0 is valid.

When perf_thread_map__pid() of index 0 is read then return the expected
"any" -1 value.

Assert this is only done for index 0.

Signed-off-by: Ian Rogers <[email protected]>
Acked-by: Gautam Menghani <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Howard Chu <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Madhavan Srinivasan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: captain5050

tools/lib/perf/threadmap.c

@@ -97,5 +97,10 @@ int perf_thread_map__nr(struct perf_thread_map *threads)
 
 pid_t perf_thread_map__pid(struct perf_thread_map *map, int idx)
 {
+	if (!map) {
+		assert(idx == 0);
+		return -1;
+	}
+
 	return map->map[idx].pid;
 }