[Patch] Fix libstdc++/9533

[Paolo Carlini]

Nathan Myers wrote:

>One more thing: for a stateful encoding, the most you can return 
>is 0 unless you have converted characters already in the buffer.  
>The pending sequence might be just shift and unshift prefixes with 
>no actual character.  (The return value is the sum of whatever is 
>already in the buffer and what you get using recv/fstat.  You only
>return -1 if the buffer is empty *and* you're sure there's nothing
>else to come.)
>
Thanks for the addtional clarifications: for sure I would have done 
something wrong otherwise.

Paolo.