Fix for ONOS-5306:NPE in Flows Instructions Decoder

Change-Id: I708a074c927841577b4eaf0ffe75ec98fac227f6
(cherry picked from commit af08dcdd4459eb47f9a995aef36674830641a8d3)
diff --git a/core/common/src/main/java/org/onosproject/codec/impl/InstructionCodec.java b/core/common/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
index 9da240d..3078fa8 100644
--- a/core/common/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
+++ b/core/common/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
@@ -62,6 +62,8 @@
 
     protected static final String MISSING_MEMBER_MESSAGE =
             " member is required in Instruction";
+    protected static final String ERROR_MESSAGE =
+            " not specified in Instruction";
 
 
     @Override