[Patch] Fix libstdc++/10097

[Paolo Carlini]

Hi,

the below fixes the problem essentially by unconditionally
doing:

    if (gptr() < egptr())
      return *gptr();

on top of underflow, as suggested by Pétur and favored
by Nathan.

Tested x86-linux. If approved, seems good for 3.3 too.

In general, my plans are as follows:

  1- Have in fixes for 9533 and 10097: proposals already
     posted.
  2- Have in Pétur's patches for 9964, 9581/9870: both
     ok with me!
  3- Return to 9701 to fix the second part: seems easy in
     the light of some obervations from Nathan.
  4- Finally return to 9423/9024, and then 9875/9178.

Nathan? Benjamin?

Thanks,
Paolo.

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