[wasm-gc] Fix ClusterFuzz issues

Random-generated modules can take surprising code paths.

Bug: chromium:1072127, chromium:1072115
Change-Id: Id9973ebe5942e95e6006026c8cbf875d826d355a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2156765Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67243}

src/wasm/module-decoder.cc

@@ -1726,7 +1726,10 @@ class ModuleDecoderImpl : public Decoder {
               break;
             case kLocalI31Ref:
             case kLocalRttRef:
-              UNIMPLEMENTED();  // TODO(7748): Add i31ref and rttref.
+              if (enabled_features_.has_gc()) {
+                UNIMPLEMENTED();  // TODO(7748): implement.
+              }
+              break;
             default:
               break;
           }

src/wasm/wasm-opcodes.cc

@@ -369,6 +369,10 @@ const char* WasmOpcodes::OpcodeName(WasmOpcode opcode) {
       return "unknown";
     // clang-format on
   }
+  // Even though the switch above handles all well-defined enum values,
+  // random modules (e.g. fuzzer generated) can call this function with
+  // random (invalid) opcodes. Handle those here:
+  return "invalid opcode";
 }
 
 #undef CASE_OP