Don't check index ram usage if index service is non-active.
Don't check index ram usage if index service is non-active.

For example, when node is failed over, there won't be a stats_reader
for @index. So menelaus_web_alerts_srv will be crashing continuously.

Change-Id: I94dae3e9b6fc9db45ede3267f02f5bb56a9da044
Reviewed-on: http://review.couchbase.org/60215
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server