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

[Paolo Carlini]

Pétur Runólfsson wrote:

>>the below fixes the PR by using Nathan's nice advice:
>>    
>>
>Great! Unless I'm mistaken, this means that the *_pback_* members
>are no longer used by basic_streambuf, and can be moved to
>basic_filebuf, where they belong.
>
Indeed, it looks like you are right (it's also clearly spelled in the 
comment I patched yesterday...).

I will work on that this evening.

Thanks!
Paolo.