Fix a minor crash-on-invalid.

llvm-svn: 55082
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 29a2429..9eccd06 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -742,7 +742,7 @@
     ExprResult Expr = ParseAssignmentExpression();
     if (Expr.isInvalid) {
       SkipUntil(tok::r_paren);
-      return Res;
+      return ExprResult(true);
     }
 
     if (ExpectAndConsume(tok::comma, diag::err_expected_comma, "",tok::r_paren))