Fix ep_engine crashing error
Fix ep_engine crashing error

When using cli to start/stop persistence for ep_engine, whole
memcached process crashing due to NULL pointer error. We need to
detect it before using strlen.

Change-Id: I38796149510b6b1e8ccdf614055f4f618c423867
Reviewed-on: http://review.membase.org/5123
Tested-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

couchbase/ep-engine,couchbase/ep-engine,membase/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,sriganes/ep-engine,hisundar/ep-engine,owendCB/ep-engine,abhinavdangeti/ep-engine,daverigby/ep-engine,membase/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,zbase/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,membase/ep-engine,owendCB/ep-engine,abhinavdangeti/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,zbase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,hisundar/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,daverigby/kv_engine,couchbaselabs/ep-engine,zbase/ep-engine,daverigby/ep-engine,zbase/ep-engine,teligent-ru/ep-engine,couchbaselabs/ep-engine,daverigby/ep-engine,daverigby/kv_engine,hisundar/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,zbase/ep-engine