perf stat: Move target check to loop control statement

perf stat: Move target check to loop control statement

Check for target existence in loop control statement jointly external
asynchronous 'done' signal.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/79037528-578c-af64-f06c-a644b7f5ba6a@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>