[Patch] istreambuf_iterator::operator++ simplification

[Paolo Carlini]

Hi,

in private email, Nathan very kindly gave a lot of interesting suggestions
for further improving and streamlining istreambuf_iterator; I'm separately
and incrementally testing and benchmarking each one.

The below just notices that both operator++ can avoid checking the return
value of sbumpc() and it's worth a small but consistent improvement in
in many different tests.

Regtested x86-linux, will wait 'til this evening (italian time) in case
of comments.

Paolo.

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