Bug Fixed load_balance exercise (p4lang#712)

Signed-off-by: Jinhua Guo <jinhua@UMICH.EDU>

Author: guojinhua-umd

exercises/load_balance/load_balance.p4

@@ -136,8 +136,9 @@ control MyIngress(inout headers hdr,
         /* TODO: apply ecmp_group table and ecmp_nhop table if IPv4 header is
          * valid and TTL hasn't reached zero
          */
-        ecmp_group.apply();
-        ecmp_nhop.apply();
+        if (ecmp_group.apply().hit) { 
+            ecmp_nhop.apply(); 
+        }
     }
 }
 

exercises/load_balance/solution/load_balance.p4

@@ -141,8 +141,9 @@ control MyIngress(inout headers hdr,
     }
     apply {
         if (hdr.ipv4.isValid() && hdr.ipv4.ttl > 0) {
-            ecmp_group.apply();
-            ecmp_nhop.apply();
+            if (ecmp_group.apply().hit) { 
+                ecmp_nhop.apply(); 
+            }
         }
     }
 }