ORA-32766：instr with negative offset

错误原因：The current varchar behavior is different. e.g. instr('abcd', 'cd', -2, 1) returns 3, whereas instr(to_clob('abcd'), 'cd', -2, 1) returns 0, (i.e. no match), because the reverse search starts from offset -2, which points to 'c' and moving backward, i.e. 'd' is ignored. This is symmetric to instr('dcba','dc',2,1), which returns 0.