Clear statement before closing db in cookie code.

sql/statement.h

Index: sql/statement.h
diff --git a/sql/statement.h b/sql/statement.h
index c7e2c40542f90f00b71c5a84d31ea5a89e4c82d4..0c58e3a9fe1745965496f8dc5e3ce6a35e659d5d 100644
--- a/sql/statement.h
+++ b/sql/statement.h
@@ -55,6 +55,10 @@ class SQL_EXPORT Statement {
   // be valid. Use is_valid() to check if it's OK.
   void Assign(scoped_refptr<Connection::StatementRef> ref);
 
+  // Resets the statement to an uninitialized state corrosponding to
+  // the default constructor, releasing the StatementRef.
+  void Clear();
+
   // Returns true if the statement can be executed. All functions can still
   // be used if the statement is invalid, but they will return failure or some
   // default value. This is because the statement can become invalid in the