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

[Benjamin Kosnik]

> 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. 

You can queue for 3.1.1 though.

-benjamin