reset spm ctx before batched multi stmt fallback to execute one by one

src/sql/ob_sql_context.h

@@ -416,6 +416,7 @@ struct ObSpmCacheCtx
   ObSpmCacheCtx()
     : bl_key_()
   {}
+  inline void reset() { bl_key_.reset(); }
   ObBaselineKey bl_key_;
 };