perf record: Fix a segfault in record__read_lost_samples()

When it fails to open events record__open() returns without setting the
session->evlist. Then it gets a segfault in the function trying to read
lost sample counts. You can easily reproduce it as a normal user like:

$ perf record -p 1 true
...
perf: Segmentation fault
...

Skip the function if it has no evlist. And add more protection for evsels
which are not properly initialized.

Fixes: a49aa8a54e861af1 ("perf record: Read and inject LOST_SAMPLES events")
Signed-off-by: Namhyung Kim <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Leo Yan <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-record.c

···
 	struct perf_record_lost_samples *lost;
 	struct evsel *evsel;
 
+	/* there was an error during record__open */
+	if (session->evlist == NULL)
+		return;
+
 	lost = zalloc(PERF_SAMPLE_MAX_SIZE);
 	if (lost == NULL) {
 		pr_debug("Memory allocation failed\n");
···
 	evlist__for_each_entry(session->evlist, evsel) {
 		struct xyarray *xy = evsel->core.sample_id;
 
+		if (xy == NULL || evsel->core.fd == NULL)
+			continue;
 		if (xyarray__max_x(evsel->core.fd) != xyarray__max_x(xy) ||
 		    xyarray__max_y(evsel->core.fd) != xyarray__max_y(xy)) {
 			pr_debug("Unmatched FD vs. sample ID: skip reading LOST count\n");