perf stat: Fix NULL pointer dereference

If config->aggr_map is NULL and config->aggr_get_id is not NULL,
the function print_aggr() will still calling arrg_update_shadow(),
which can result in accessing the invalid pointer.

Fixes: 088519f3 ("perf stat: Move the display functions to stat-display.c")
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wei Li <liwei391@huawei.com>
Link: https://lore.kernel.org/lkml/20200608163625.GC3073@kernel.org


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