Codec error handling callback should return newpos > start

According to PEP 293, the codec error handling callback should either raise an exception or return a pair (replacement, newpos). newpos specifies a new position within object, where the encoder/decoder will continue encoding/decoding. Negative values for newpos are treated as being relative to end of object. PEP 293 does not specify the bound of newpos, and this is a problem.

If newpos == start, the codec will not advance. If replacement is empty, it will just hang in an infinite loop, calling the callback again and again. If replacement is not empty, it will additionally add it in an internal buffer, and finally, after long time, you will get a MemoryError. If newpos < start, the behavior may be even more weird.

Before fixing other errors in #81000 it even caused a crash, but now it is only an infinite loop.

I propose to limit the returning value by range [start+1, len(object)].

@malemburg, @doerwalter, @vstinner

先閱讀 PEP 293，並追蹤 issue 中描述的 encoder/decoder error-callback 行為。調查傳回的 newpos 值等於或小於 start 時的處理方式，然後為回報的迴圈與無效位置案例新增 coverage。完成的標準是這些 callbacks 不再造成處理不前進或無限緩衝。