[wasm] Fix validation error for missing return statement in asm.js module.

src/typing-asm.cc

Index: src/typing-asm.cc
diff --git a/src/typing-asm.cc b/src/typing-asm.cc
index 6e7b90ccd120f7a30d721b570d375d7d6979d927..b969d45166586ae35feb774bee642f6662c0e52a 100644
--- a/src/typing-asm.cc
+++ b/src/typing-asm.cc
@@ -127,6 +127,9 @@ void AsmTyper::VisitAsmModule(FunctionLiteral* fun) {
 
   // Validate exports.
   ReturnStatement* stmt = fun->body()->last()->AsReturnStatement();
+  if (stmt == nullptr) {
+    FAIL(fun->body()->last(), "last statement in module is not a return");
+  }
   RECURSE(VisitWithExpectation(stmt->expression(), Type::Object(),
                                "expected object export"));
 }