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 <leo.yan@linaro.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20190708143937.7722-2-leo.yan@linaro.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/browsers/hists.c

@@ -639,6 +639,10 @@ int hist_browser__run(struct hist_browser *browser, const char *help,
 		switch (key) {
 		case K_TIMER: {
 			u64 nr_entries;
+
+			WARN_ON_ONCE(!hbt);
+
+			if (hbt)
 				hbt->timer(hbt->arg);
 
 			if (hist_browser__has_filter(browser) ||
@@ -2821,7 +2825,7 @@ static int perf_evsel__hists_browse(struct perf_evsel *evsel, int nr_events,
 {
 	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];
@@ -3087,6 +3091,8 @@ static int perf_evsel__hists_browse(struct perf_evsel *evsel, int nr_events,
 			goto skip_annotation;
 
 		if (sort__mode == SORT_MODE__BRANCH) {
+
+			if (browser->he_selection)
 				bi = browser->he_selection->branch_info;
 
 			if (bi == NULL)
@@ -3271,6 +3277,7 @@ static int perf_evsel_menu__run(struct perf_evsel_menu *menu,
 
 		switch (key) {
 		case K_TIMER:
+			if (hbt)
 				hbt->timer(hbt->arg);
 
 			if (!menu->lost_events_warned &&