perf report: Fix segmentation fault when running with '-g none'

perf report: Fix segmentation fault when running with '-g none'

Segmentation fault occurs when running perf report with '-g
none'.

Reported-by: Austin Zhang <austin.zhang@intel.com>
Signed-off-by: Yong Wang <yong.y.wang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20100122014750.GA4111@ywang-moblin2.bj.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>