check that ipv6 header is valid before using

This currently causes a crash in x4c compiled code. A good question is
what should happen here. According to P4 spec x4c should emit an
unspecified value. According to ry we should apport the pipeline.

- #6

Perhaps we can catch some of these at compile time?

- #7

test/src/p4/sidecar-lite.p4

@@ -420,13 +420,13 @@ control local(
 
     apply {
         local.apply();
-
-        bit<16> ll = 16w0xff02;
-
-        //TODO this is backwards should be
-        //if(hdr.ipv6.dst[127:112] == ll) {
-        if(hdr.ipv6.dst[15:0] == ll) {
-            is_local = true;
+        if(hdr.ipv6.isValid()) {
+            bit<16> ll = 16w0xff02;
+            //TODO this is backwards should be
+            //if(hdr.ipv6.dst[127:112] == ll) {
+            if(hdr.ipv6.dst[15:0] == ll) {
+                is_local = true;
+            }
         }
     }