perf stat: Fix aggreate counter reading accounting

Introduced in: c52b12ed, when this sequence:

  count[0] = count[1] = count[2] = 0;

Was replaced with:

  aggr->val = 0;

Which is equivalent to zeroing just the first entry in the 'count'
array.

Fix it by zeroing the three entries with:

  aggr->val = aggr->ena = aggr->run = 0;

Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>