perf report TUI: Fix segmentation fault in perf_evsel__hists_browse()

The segmentation fault can be reproduced as following steps:

1) Executing perf report in tui.

2) Typing '/xxxxx' to filter the symbol to get nothing matched.

3) Pressing enter with no entry selected.

Then it will report a segmentation fault.

It is caused by the lack of check of browser->he_selection when
accessing it's member res_samples in perf_evsel__hists_browse().

These processes are meaningful for specified samples, so we can skip
these when nothing is selected.

Fixes: 4968ac8 ("perf report: Implement browsing of individual samples")
Signed-off-by: Wei Li <liwei391@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: http://lore.kernel.org/lkml/20200612094322.39565-1-liwei391@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/browsers/hists.c

@@ -2288,6 +2288,11 @@ static struct thread *hist_browser__selected_thread(struct hist_browser *browser
 	return browser->he_selection->thread;
 }
 
+static struct res_sample *hist_browser__selected_res_sample(struct hist_browser *browser)
+{
+	return browser->he_selection ? browser->he_selection->res_samples : NULL;
+}
+
 /* Check whether the browser is for 'top' or 'report' */
 static inline bool is_report_browser(void *timer)
 {
@@ -3357,16 +3362,16 @@ static int perf_evsel__hists_browse(struct evsel *evsel, int nr_events,
 					     &options[nr_options], NULL, NULL, evsel);
 		nr_options += add_res_sample_opt(browser, &actions[nr_options],
 						 &options[nr_options],
-				 hist_browser__selected_entry(browser)->res_samples,
-				 evsel, A_NORMAL);
+						 hist_browser__selected_res_sample(browser),
+						 evsel, A_NORMAL);
 		nr_options += add_res_sample_opt(browser, &actions[nr_options],
 						 &options[nr_options],
-				 hist_browser__selected_entry(browser)->res_samples,
-				 evsel, A_ASM);
+						 hist_browser__selected_res_sample(browser),
+						 evsel, A_ASM);
 		nr_options += add_res_sample_opt(browser, &actions[nr_options],
 						 &options[nr_options],
-				 hist_browser__selected_entry(browser)->res_samples,
-				 evsel, A_SOURCE);
+						 hist_browser__selected_res_sample(browser),
+						 evsel, A_SOURCE);
 		nr_options += add_switch_opt(browser, &actions[nr_options],
 					     &options[nr_options]);
 skip_scripting: