Add check on rebalance interval (#5544) (#5595)

(cherry picked from commit fa5c3a3)

dgraph/cmd/zero/run.go

@@ -181,6 +181,12 @@ func run() {
 			return true, true
 		}
 	}
+
+	if opts.rebalanceInterval <= 0 {
+		log.Fatalf("ERROR: Rebalance interval must be greater than zero. Found: %d",
+			opts.rebalanceInterval)
+	}
+
 	grpc.EnableTracing = false
 	otrace.ApplyConfig(otrace.Config{
 		DefaultSampler: otrace.ProbabilitySampler(Zero.Conf.GetFloat64("trace"))})