WireCmd: error out if "self" is null.

BUG=dawn:34

Change-Id: Ic77a6a1b2145ad9572abd6e2f3cd422d8c87f3eb
Reviewed-on: https://dawn-review.googlesource.com/c/2301
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

generator/templates/dawn_wire/WireCmd.cpp

@@ -211,6 +211,14 @@
             //* guarantees they are filled even if there is an ID for an error object for the
             //* Maybe monad mechanism.
             DESERIALIZE_TRY(resolver.GetFromId(record->selfId, &record->self));
+
+            //* The object resolver returns a success even if the object is null because the
+            //* frontend is reponsible to validate that (null objects sometimes have special
+            //* meanings). However it is never valid to call a method on a null object so we
+            //* can error out in that case.
+            if (record->self == nullptr) {
+                return DeserializeResult::FatalError;
+            }
         {% endif %}
 
         {% if is_struct and as_struct.extensible %}