perf: Fix initialization bug in parse_single_tracepoint_event()

The parse_single_tracepoint_event() was setting some attributes
before it validated the event was indeed a tracepoint event. This
caused problems with other initialization routines like in the
builtin-top.c module whereby sample_period is not set if not 0.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <4bcf232b.698fd80a.6fbe.ffffb737@mx.google.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>