[RFC] libstdc++/9404 or weird stringbuf internals

[Paolo Carlini]

Thanks Nathan!

Finally, thanks to your precious advice, I have got a real fix for 9404, 
which already passes the testsuite and the new testcase! ;-)

Now I shall test it more throughly, with a small _M_buf_size_opt too, in 
order to test the behaviour for many reallocations.

At this point, however, I believe I have to take also a look to 
libstdc++/9701, which seems related.

OTOH, probably I will leave for a second round the improvements for 
str() which you suggested.

Paolo.