perf hists browser: Fix potential NULL pointer dereference found by the smatch tool

Based on the following report from Smatch, fix the potential
NULL pointer dereference check.

tools/perf/ui/browsers/hists.c:641
hist_browser__run() error: we previously assumed 'hbt' could be
null (see line 625)

tools/perf/ui/browsers/hists.c:3088
perf_evsel__hists_browse() error: we previously assumed
'browser->he_selection' could be null (see line 2902)

tools/perf/ui/browsers/hists.c:3272
perf_evsel_menu__run() error: we previously assumed 'hbt' could be
null (see line 3260)

This patch firstly validating the pointers before access them, so can
fix potential NULL pointer dereference.

Signed-off-by: Leo Yan <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Mathieu Poirier <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Suzuki Poulouse <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/ui/browsers/hists.c

···
 		switch (key) {
 		case K_TIMER: {
 			u64 nr_entries;
-			hbt->timer(hbt->arg);
+
+			WARN_ON_ONCE(!hbt);
+
+			if (hbt)
+				hbt->timer(hbt->arg);
 
 			if (hist_browser__has_filter(browser) ||
 			    symbol_conf.report_hierarchy)
···
 {
 	struct hists *hists = evsel__hists(evsel);
 	struct hist_browser *browser = perf_evsel_browser__new(evsel, hbt, env, annotation_opts);
-	struct branch_info *bi;
+	struct branch_info *bi = NULL;
 #define MAX_OPTIONS  16
 	char *options[MAX_OPTIONS];
 	struct popup_action actions[MAX_OPTIONS];
···
 			goto skip_annotation;
 
 		if (sort__mode == SORT_MODE__BRANCH) {
-			bi = browser->he_selection->branch_info;
+
+			if (browser->he_selection)
+				bi = browser->he_selection->branch_info;
 
 			if (bi == NULL)
 				goto skip_annotation;
···
 
 		switch (key) {
 		case K_TIMER:
-			hbt->timer(hbt->arg);
+			if (hbt)
+				hbt->timer(hbt->arg);
 
 			if (!menu->lost_events_warned &&
 			    menu->lost_events &&