perf hists browser: Invalidate ui_browser->top after timer calls

perf hists browser: Invalidate ui_browser->top after timer calls

With underlying dynamic data structures we need to invalidate pointers
to them after a timer, as that entry may have vanished (decayed in top,
for instance).

I forgot about browser_ui->top. Fix it by resetting it to null after a
timer. The seek operation from SEEK_SET will then set it to a valid
entry because it starts from rb_first(&hists->entries).

Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-2ssjm0ouh9tsz4dwkcu7c40n@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>