Only cleanup priv->shd.statistics if created

It is possible that the statistics array was never created and
dereferencing it may case a segfault.

BUG: 1147156
Change-Id: If905457ba985add62c3ed543bced1313640af762
Signed-off-by: Tiziano Müller <tiziano.mueller@stepping-stone.ch>
Reviewed-on: http://review.gluster.org/8873
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>