fix expr statement_digest core

src/sql/engine/expr/ob_expr_statement_digest.cpp

@@ -99,6 +99,7 @@ int calc_digest_text(ObIAllocator &allocator,
         digest_str = queries.at(i);
       }
     }
+    exec_ctx.set_physical_plan_ctx(NULL);
   }
   return ret;
 }