MDEV-10713: signal 11 error on multi-table update - crash in handler::increment_statistics or in make_select or assertion failure pfs_thread == ((PFS_thread*) pthread_getspecific((THR_PFS)))

Different fix. Don't allow Item_func_sp to be evaluated unless
all tables are prelocked.

Extend the test case to make sure Item_func_sp::val_str is called
(the table must have at least one row for that).