[sql] Complain about statement mutations after stepping has started.

sql/statement.h

Index: sql/statement.h
diff --git a/sql/statement.h b/sql/statement.h
index 5fedc53a09b1bd25c749693b33185f6ab730d465..acf89e707fef0c0f9f571ad3a29bf64616fcdda7 100644
--- a/sql/statement.h
+++ b/sql/statement.h
@@ -178,6 +178,11 @@ class SQL_EXPORT Statement {
   // guaranteed non-NULL.
   scoped_refptr<Connection::StatementRef> ref_;
 
+  // Set after Step() or Run() are called, reset by Reset().  Used to
+  // prevent accidental calls to API functions which would not work
+  // correctly after stepping has started.
+  bool stepped_;
+
   // See Succeeded() for what this holds.
   bool succeeded_;