perf top: Fix zero or negative refresh delay

If perf top is executed with a zero value for the refresh rate,
we get a division by zero exception while computing samples_per_sec.

Also a zero refresh rate is not possible, neither do we want to
accept negative values.

[ Impact: fix division by zero in perf top ]

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1244223061-5399-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>