priority 0 is off limits; because of OF13

Change-Id: I50fb8575450f11c500f532e878597d3b1632e50a
diff --git a/core/api/src/main/java/org/onlab/onos/net/flow/DefaultFlowRule.java b/core/api/src/main/java/org/onlab/onos/net/flow/DefaultFlowRule.java
index bb4805b..d489870 100644
--- a/core/api/src/main/java/org/onlab/onos/net/flow/DefaultFlowRule.java
+++ b/core/api/src/main/java/org/onlab/onos/net/flow/DefaultFlowRule.java
@@ -50,7 +50,8 @@
     public DefaultFlowRule(DeviceId deviceId, TrafficSelector selector,
             TrafficTreatment treatement, int priority, ApplicationId appId,
             int timeout) {
-        this(deviceId, selector, treatement, priority,
+
+        this(deviceId, selector, treatement, priority == 0 ? 1 : priority,
                 FlowRuleState.CREATED, appId, timeout);
     }