[v3] Fix libstdc++/16401

[Paolo Carlini]

Hi,

the below fixes a serious performance regression in 3.4/mainline: when
I tweaked Nathan's rewrite of stringbuf::overflow to spare a copy, I
forgot, my bad, that now __tmp was a *new* string and I couldn't rely
on it growing automatically exponentially from the old _M_string size.

Tested x86-linux.

Paolo.

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