perf script: Fix a NULL pointer dereference

If 'perf script --gen-script' was called with a perf.data which contains
no tracepoint event, it'd segfault due to NULL pevent pointer. Fix it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1344909423-26384-1-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>