fuzzers: Fix function not returning a value warning

This was preventing the dawn -> chromium autoroller

Change-Id: Iea260e8b454766e08cdb69cea65222391a4022bd
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58680
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>

fuzzers/tint_ast_fuzzer/mutation.cc

@@ -32,7 +32,9 @@ std::unique_ptr<Mutation> Mutation::FromMessage(
           message.replace_identifier());
     case protobufs::Mutation::MUTATION_NOT_SET:
       assert(false && "Mutation is not set");
+      break;
   }
+  return nullptr;
 }
 
 }  // namespace ast_fuzzer