perf sched: Do not delete session object prematurely

The session object is released prematurely when processing events for
latency command. The session's thread objects are used within the
output_lat_thread function.

Runnning following commands:

 # perf sched record
 # perf sched latency

the latter displays incorrect data and might cause access violation.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1312837414-3819-1-git-send-email-jolsa@redhat.com
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>