Fix NullRefrenceException. (#660)

In multi thread environment, run AutoQuery.CreateQuery method with the same From Type will cause null reference.

Author: 李孟宁

src/ServiceStack.OrmLite/Expressions/SqlExpression.cs

@@ -934,7 +934,7 @@ private SqlExpression<T> OrderByFields(string orderBySuffix, FieldDefinition[] f
             foreach (var field in fields)
             {
                 var tableDef = GetModelDefinition(field);
-                var qualifiedName = modelDef != null
+                var qualifiedName = tableDef != null
                     ? GetQuotedColumnName(tableDef, field.Name)
                     : DialectProvider.GetQuotedColumnName(field);