[System test]: Mitigate panic from go-couchbase when failover logs are requested post KV hard failover

panic: runtime error: index out of range

goroutine 69190 [running]:

panic(0xc7b820, 0xc4200160c0)

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:500 +0x1a1 fp=0xc42b4eb010 sp=0xc42b4eaf80

runtime.panicindex()

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:27 +0x6d fp=0xc42b4eb040 sp=0xc42b4eb010

github.com/couchbase/eventing/dcp.Bucket.getMasterNode(0xc423061580, 0xc42f49d5f0, 0xc42f49d638, 0xc42ae00898, 0x4, 0xc425420120, 0x8, 0x9, 0x0, 0x0, ...)

	goproj/src/github.com/couchbase/eventing/dcp/pools.go:218 +0xb5 fp=0xc42b4eb1d0 sp=0xc42b4eb040

github.com/couchbase/eventing/dcp.(*Bucket).GetFailoverLogs(0xc42c344780, 0xabcd, 0xc42ce43000, 0x400, 0x400, 0xc4292505d0, 0x0, 0x0, 0x0)

	goproj/src/github.com/couchbase/eventing/dcp/upr.go:93 +0x19e fp=0xc42b4eb658 sp=0xc42b4eb1d0

github.com/couchbase/eventing/consumer.glob..func20(0xc42f0d75c0, 0x3, 0x3, 0x0, 0x0)

	goproj/src/github.com/couchbase/eventing/consumer/bucket_ops.go:836 +0x4bd fp=0xc42b4eb7e8 sp=0xc42b4eb658

github.com/couchbase/eventing/util.Retry(0x154a8a0, 0xc428f693f0, 0xc4219db258, 0xe418b8, 0xc42f0d75c0, 0x3, 0x3, 0x0, 0x0)

	goproj/src/github.com/couchbase/eventing/util/retry.go:65 +0x63 fp=0xc42b4eb858 sp=0xc42b4eb7e8

github.com/couchbase/eventing/consumer.(*Consumer).handleFailoverLog(0xc4219df400)

	goproj/src/github.com/couchbase/eventing/consumer/process_events.go:1136 +0x8e3 fp=0xc42b4ebf88 sp=0xc42b4eb858

runtime.goexit()

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/asm_amd64.s:2086 +0x1 fp=0xc42b4ebf90 sp=0xc42b4ebf88

created by github.com/couchbase/eventing/consumer.(*Consumer).Serve

	goproj/src/github.com/couchbase/eventing/consumer/v8_consumer.go:237 +0x968