perf hists browser: Fix NULL deref in hists browsing code

If there's an event with no samples in data file, the perf report
command can segfault after entering the event details menu.

Following steps reproduce the issue:

 # ./perf record -e syscalls:sys_enter_kexec_load,syscalls:sys_enter_mmap ls
 # ./perf report
 # enter '0 syscalls:sys_enter_kexec_load' menu
 # pres ENTER twice

Above steps are valid assuming ls wont run kexec.. ;)

The check for sellection to be NULL is missing. The fix makes sure it's
being check. Above steps now endup with menu being displayed allowing
'Exit' as the only option.
Signed-off-by: NJiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1333570898-10505-2-git-send-email-jolsa@redhat.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/ui/browsers/hists.c

@@ -125,6 +125,9 @@ static int callchain__count_rows(struct rb_root *chain)
 
 static bool map_symbol__toggle_fold(struct map_symbol *self)
 {
+	if (!self)
+		return false;
+
 	if (!self->has_children)
 		return false;