[Patch] Fix in_avail half of libstdc++/9701

[Paolo Carlini]

Hi,

the below fixes the PR by using Nathan's nice advice:

    http://gcc.gnu.org/ml/libstdc++/2003-03/msg00070.html

(see also the final part of:

    http://gcc.gnu.org/ml/libstdc++/2003-03/msg00066.html

for further explanations)

Tested x86-linux. Benjamin, do you like it?

Paolo.

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