SIGSEGV: Service 'kv' exited with status 2

Steps to reproduce:

1. Create a dataplane
2. Create 20 databases. Wait for kv auto-scaling from 3->6
3. Update width for all dbs from 1->2
4. Update width for 1 db from 2->3 and wait for kv auto-scaling
5. For some reason, kv nodes are scaled to 12 instead of 9. Have to figure out why and log a new ticket on CP.
6. As the test failed, it started deleting all the 20 databases. KV is seen crashing due to SIGSERV is regulator. This is seen on all 12 KV nodes:

   Service 'kv' exited with status 2. Restarting. Messages:

   [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb98bc0]

   goroutine 387 [running]:

   github.com/couchbase/regulator/impl.(*storageLimitingImpl).refreshStorageStats.func1(0x40004a4900?, 0x40000120f0, 0x4000ad0e28?, 0x40004a4900)

   /home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/regulator/impl/storagelimiting.go:237 +0x5a0

   github.com/couchbase/regulator/impl.(*storageLimitingImpl).refreshStorageStats(0x40004a4900, 0x4000ad0f80?, 0x0?)

   /home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/regulator/impl/storagelimiting.go:241 +0x100

   github.com/couchbase/regulator/impl.(*storageLimitingImpl).storageReportGenerator.func1()

   /home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/regulator/impl/storagelimiting.go:160 +0xa8

   created by github.com/couchbase/regulator/impl.(*storageLimitingImpl).storageReportGenerator

   /home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/regulator/impl/storagelimiting.go:148 +0xac