[3.1.9 backport] Panic in CheckpointHash function for bucket UUID call

panic: runtime error: invalid memory address or nil pointer dereference

[signal SIGSEGV: segmentation violation code=0x1 addr=0x68 pc=0xeb693c]

goroutine 900832 [running]:

github.com/couchbase/sync_gateway/db.ActiveReplicatorConfig.CheckpointHash({{0xc4725c3590, 0x18}, {0x0, 0x0}, {0x0, 0x0, 0x0}, {0x0, 0x0, 0x0}, ...}, ...)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_config.go:163 +0x39c

github.com/couchbase/sync_gateway/db.(*ActivePullReplicator)._initCheckpointer.func1(0xc708afa1e0)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_pull.go:200 +0xdb

github.com/couchbase/sync_gateway/db.(*activeReplicatorCommon).forEachCollection(0x1731100?, 0xc7375d2b40?)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_common_collections.go:194 +0x76

github.com/couchbase/sync_gateway/db.(*ActivePullReplicator)._initCheckpointer(0xc7f20eb640, {0x0, 0x0, 0x0})

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_pull.go:199 +0xd5

github.com/couchbase/sync_gateway/db.(*ActivePullReplicator)._startPullNonCollection(0xc7f20eb640)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_pull.go:117 +0xc5

github.com/couchbase/sync_gateway/db.(*ActivePullReplicator)._connect(0xc7f20eb640)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_pull.go:90 +0x28a

github.com/couchbase/sync_gateway/db.(*activeReplicatorCommon).reconnectLoop.func2()

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_common.go:186 +0x14a

github.com/couchbase/sync_gateway/base.RetryLoop({0x1731100, 0xc7375d2b40}, {0x13554b4, 0x14}, 0xc6f9daff90, 0xc59c971c80)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/base/util.go:450 +0x7a

github.com/couchbase/sync_gateway/db.(*activeReplicatorCommon).reconnectLoop(0xc708aea9a0)

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_common.go:198 +0x285

created by github.com/couchbase/sync_gateway/db.(*ActivePullReplicator).Start

	/home/couchbase/jenkins/workspace/sgw-unix-build/3.1.2/enterprise/sync_gateway/db/active_replicator_pull.go:65 +0x397