[Patch] Fix libstdc++/13858

[Paolo Carlini]

Hi,

the fix is straighforward: consistently with underflow, throw an
exception from overflow too (actually, _M_convert_to_external) in
case of conversion errors. Nice that overall the affected code
becomes simpler.

The new behavior implies that I had to tweak a few pre-existing
tests which more naively checked for eof in case of failure.

Took the occasion to simplify a trivial conditional.

Tested x86-linux. If nobody objects will commit to both mainline
and 3_4-branch before the end of the week.

Paolo.

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