perf top: Fix --delay_secs 0 division by zero

Add delay_secs sanity check to handle_keypress,
this fixes a division by zero crash.

Signed-off-by: Tim Blechmann <tim@klingt.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4AD9EBFD.106@klingt.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>