[v3, 3.2 and 3.1] Fix replace regression wrt 2.95.x and 3.0.x

[Paolo Carlini]

Benjamin Kosnik wrote:

>>as I did already anticipate yesterday, the fix for
>>__throw_length_error in basic_string::replace turns out to be quite
>>critical, since not only tightens the check but avoid throwing in the
>>/wrong/ situations. I have added therefore a testcase which represents
>>a regression wrt 2.95.x and 3.0.x.
>>    
>>
>This is fine for mainline Paolo. For the branch, you'll probably have to
>wait for 3.1.1, unless Mark approves. Right now, I think he's only
>approving fixes to previously reported, GNATS bugs.
>
Ok, thanks!

Mark? ;-)

Ciao, Paolo.